what are angles grade 4

Classify the angle in the diagram below: The vertex of ABC is where the rays meet at the square symbol, which means that the angle is 90^{\circ} and a right angle. O is the vertex in the given figure. . Reflex Angle. problem and check your answer with the step-by-step explanations. Similarly, 360000000 is coterminal with the zero angle and the full angle, but it represents spinning all the way around 1 million times. Did you know? An angle that turns throughnone-degree angles is said to have an angle measure ofndegrees. However, they can excel in geometry with fun games on SplashLearn! Browse all of our geometry worksheets, from the basic shapes through areas and perimeters, angles, grids and 3D shapes. This content is available to members only. Parts of an Angle. Unlike algebra, geometry is more artistic than analytical. There are four important types of angle: acute, right, obtuse, and reflex. Join K5 to save time, skip ads and access more content. But the full angle represents spinning around all the way one time, whereas the zero angle represents not spinning around at all. In addition, motivating kids to practice all types of angles and their measurements will help you improve their knowledge of the topic. Lesson Plans and Worksheets for Grade 4 Dive deep into the world of geometry by selecting the shape that fulfills the given conditions. . Why might Nick have thought the sections of his circle would have a larger angle measurement than the sections in Matthews circle? Recall the definitions of the types of angles. We welcome your feedback, comments and questions about this site or page. Provide mental stimulation to your kid with SplashLearn's angle games. A full circle has 360. The lesson contains many varied exercises for students. And the two straight sides are called arms. Choose from hundreds of topics including multiplication, division, fractions, angles, and more. The two arms of a right angle are perpendicular. 5. Enjoy the marvel of mathematics by exploring how to measure angles using a protractor. You will learn how to identify angles and classify them. Straight angles A straight angle is 180^{\circ} and it forms a straight line. Quiz yourself on shapes Acute angle Right angle Obtuse angle Reflex angle Center point Half turn Full turn This means that the two rays or lines that meet at the vertex point form a right angle. Comparison word problems: addition or multiplication? We also use third-party cookies that help us analyze and understand how you use this website. Plot points on a coordinate grid - 1st quadrant only, Reading points on a coordinate grid - 1st quadrant only, Plot points on a coordinate grid - 4 quadrants, Reading points on a coordinate grid - 4 quadrants. If you see an angle of 720, it means it has rotated twice around a full circle. An angle greater than 180 and less than 360 is called a reflex angle. Angles that measure 91, 100, 140, and 170 are some examples of obtuse angles. Learn More, Perimeter and area of irregular rectangular shapes. An angle that turns through 1/360 of a circle is called a one-degree angle, and can be used to measure angles. Enjoy the marvel of math-multiverse by learning to measure angles in multiples of 10 degrees. He divided his circle into six equal sections and measured the angles of each section. . There are two main ways to label angles: 1. give the angle a name, usually a lower-case letter like a or b, or sometimes a Greek letter like (alpha) or (theta) Please read our, Example 1: classify angle given a diagram, Example 2: classify angle given a diagram, Example 3: classify angle given a diagram, Example 4: classify angle given a measure, Example 5: classify angle given a measure. It is sometimes referred to as a full angle. A right angle is exactly 90. Angles in a circle add up to 360. KINDS OF ANGLE | GRADE 4 Sheena Doria 49.7K subscribers Subscribe 168 Share 13K views 11 months ago Objective: This video aims to help you describe and illustrate different kinds of angle. These fun games ensure that your 4th-graders understand the topic properly, giving them the right skills for advanced geometry in further grades. These things are made up of acute angles. But opting out of some of these cookies may affect your browsing experience. Begin the exciting journey of becoming a math wizard by learning how to add the angles. There are four important types of angle: acute, right, obtuse, and reflex. He expected them to be larger than 60, but they all measured 60. Learn to solve problems by drawing angles nearest to 5 and 1 degrees. Here are the best fun angle games for your kids available online: This game encourages kids to solve a series of geometry questions and determine if the angle specified in a particular form is a right, acute, or obtuse angle. JKL is 180^{\circ} , which means it is a straight angle. The corner point of an angle is called the vertex. [FREE] End of Year Math Assessments (Grade 4 and Grade 5). Necessary cookies are absolutely essential for the website to function properly. Matthew drew a small circle and divided it into six equal sections. Kids must draw angles using a protractor to practice geometry. B. Learn More. If you can, please consider purchasing a membership ($24/year) to support our efforts. Kids in grade 4 often find angles tricky to learn. If it's less than 90, then it's an acute angle. Supplementary angles are a pair of angles whose sum is 180^{\circ} and complementary angles are a pair of angles whose sum is 90^{\circ}. These cookies do not store any personal information. Assess math progress for the end of grade 4 and grade 5 or prepare for state assessments with these mixed topic, multiple choice questions and extended response questions! Right angles A right angle is 90^{\circ} . right, obtuse, and straight angles 2. Common Core: 4.MD.5. Improve your math knowledge with free questions in "Acute, right, obtuse, and straight angles" and thousands of other math skills. Extend this video lesson with unique worksheets, activities, and quizzes only available at https://learnmore.numberock.com/free-angThank you for watching our. Kids may find it difficult to practice geometry. KY.4.G.2 Classify two-dimensional figures based on the presence or absence of parallel or . I can identify the parts of an angle and define what an angle is. The game allows students to practice measuring angles using the provided tool. RST is 170^{\circ} , which is greater than 90^{\circ} and less than 180^{\circ} , so it is an obtuse angle. Created by Sal Khan. Matthew and Nick were investigating angles and circles, drawing circles and creating angles inside of their circles. If you have an acute pain, it's a sharp pain. Practice the superpower of geometry by learning how to identify benchmark angles. Embedded content, if any, are copyrights of their respective owners. Why do the sections in Nicks circle and the sections in Matthews circle have the same angle measurement? Videos, examples, solutions, and lessons to help Grade 4 students learn to recognize angles as geometric shapes that are formed wherever two rays share a common endpoint, and understand concepts of angle measurement. For example, 45 degrees is represented as 45 Parts of Angles Vertex: A vertex is a corner of an angle, a point where two lines/sides meet. Can you think of other things that have the right angles? An Introduction to Angles Worksheet for Grade 4, Practice on the Introduction to Angles Grade 4 Worksheet, Classification of Angles 4th Grade Worksheet, Practice Problems on Classification of Angles Worksheet for 4th Grade, Measurement of Angles Worksheet for Grade 4, Adding and Subtracting Fractions Worksheets 4th Grade, Partial Quotients Division 4th Grade Worksheets. Before starting, let's find out what an angle is! This fourth grade geometry lesson teaches the definitions for a line, ray, angle, acute angle, right angle, and obtuse angle. problem solver below to practice various math topics. Have students draw and create the different types of angles using colored pencils. IXL's SmartScore is a dynamic measure of progress towards mastery, rather than a percentage grade. They are measured in degrees, which are represented by the symbol . Activity 2: Since ABC is less than 90^{\circ} it must be an acute angle. The resource sheet Circles and Angles shows the work that Matthew and Nick did. An acute angle is an angle whose measure is greater than 0^{\circ} and less than 90^{\circ} . Shine bright in the math world by learning how to identify angles by their types. Worksheets are Geometry two dimensional shapes angles symmetry grade 4, Name class score what type of angles are these, Complete angle, Types of angles, Types of angles, Types of angles, Geometry part 1 lines and angles, Identify different types of angles daily life examples. KMN is a straight angle because it forms a straight line so it is equal to 180^{\circ}. IXL offers hundreds of fourth grade math skills, lessons, and games to explore and learn! So, all angles have coterminal angles by adding some multiple of 360 to them. Sample Grade 4 Geometry Worksheet More geometry worksheets Browse all of our geometry worksheets, from the basic shapes through areas and perimeters, angles, grids and 3D shapes. Great job learning about the five types of angles. Become a memberto access additional content and skip ads. Obtuse angles An obtuse angle is an angle greater than 90^{\circ} but less than 180^{\circ} . Step 3: To get the unknown angle of x, the first thing to do is to add 30 and 89. One of the benefits of angles worksheets 4th grade is that it provides an understanding of basic concepts like naming angles, identifying the parts of an angle, classifying angles, measuring angles using a protractor, angles formed between intersecting lines, or angles formed in 2D shapes, etc. An angle that is greater than 90 but less than 180 is called an obtuse angle. These worksheets are available to members only. Angle Definition Angle Definition Geometry is a branch of mathematics that deals with the study of shapes and their measurements. However, in elementary and middle school, you will only work with positive angles, and negative angle measurements are what you will encounter in high school. Right angles measure 90 degrees. Find out how we can help your students achieve success with our Elementary math tutoring programs. You also have the option to opt-out of these cookies. Paola, Teacher. out of 100. Complete turn Also known as a full rotation is 360^{\circ}. The angle is equal to 90^{\circ} , so it is a right angle. trading name of Virtual Class Ltd. Just don't start calling your family members obtuse. 4. We work with schools, teachers and universities around the world, producing customized learning experiences for children and adults of all ages: online courses, interactive textbooks, responsive websites, and other fun things. Right angles are often shown with a box in the corner. Tip: The word obtuse can describe someone who is slow to learn, or not that sharp. It tracks your skill level as you tackle progressively more difficult questions. K5 Learning offers free worksheets, flashcardsand inexpensiveworkbooksfor kids in kindergarten to grade 5. PQR is 208^{\circ} is greater than 180^{\circ} and less than 360^{\circ} , so it is a reflex angle. 5. A ray is part of a line. Acute means "sharp". These cookies will be stored in your browser only with your consent. K5 Learning offers free worksheets, flashcardsand inexpensiveworkbooksfor kids in kindergarten to grade 5. At Third Space Learning, we specialize in helping teachers and school leaders to provide personalized math support for more of their students through high-quality, online one-on-one math tutoring delivered by subject experts. With the right tools and support, your 4th-grader can understand angles better. Angles. With the guidance of these worksheets, students will be able to practice questions based on these concepts and build a clear understanding. Our members helped us give away millions of worksheets last year. The word angle comes from the Latin word angulus, which means a corner. 1.6M subscribers 153K views 2 years ago Jeevandeep PPS Mathematics WB - Std 5 Angles | Mathematics Grade 4 | Periwinkle Watch our other videos: Show more Show more Try YouTube Kids Learn. We also study how the size of the angle is ONLY determined by how much it has "opened" as compared to the whole circle. The term "angle" refers to the measurement of the "opening" between these two lines. 700 Possible mastery points Skill Summary Understanding angle measurement Quiz 1: 5 questions Practice what you've learned, and level up on the above skills Measuring angles Decomposing angles Input/output tables with addition, subtraction, multiplication, and division, Addition, subtraction, multiplication, and division word problems, Estimate sums, differences, products, and quotients: word problems, Word problems with extra or missing information, Solve word problems using guess-and-check, Multi-step word problems involving subtraction, Multi-step word problems with strip diagrams, Use strip diagrams to represent and solve multi-step word problems, Multi-step word problems involving remainders, Multi-step word problems: identify reasonable answers, Choose numbers with a particular sum, difference, product, or quotient, Mentally add and subtract numbers ending in zeroes, Write numerical expressions: one operation, Perform multiple operations with whole numbers, Inequalities involving addition, subtraction, multiplication, and division, Write variable expressions: word problems, Write variable equations to represent word problems, Create and interpret line plots with fractions, Find two numbers based on sum and difference, Find two numbers based on sum, difference, product, and quotient, Find the next row in a growing pattern of shapes. Use digital platforms for students to practice with different types of angles. Have students create designs using the different types of angles. Common Core For Grade 4. B. Classify angles and triangles as obtuse, acute, or right. QRS is 180^{\circ}, what type of angle is it? An angle is formed between two lines (rays) meeting around a common point. What is K5? Angles that measure 10, 20, 45, and 70 are all acute angles. I can explain that an angle is measured in degrees. This video relates to Common Core Standard 4.MD.5. Obtuse angles measure more than 90 degrees. (0406.4.3) Links verified on 11/10/2011. 30 + 89= 119. A full circle has 360. 6. Members skip ads and access exclusive features. 119 + x= 180. Two-Dimension Shapes, Angles, and Symmetry Grade: 4 Mathematical goals This lesson is intended to help you assess how well students are able to: . An angle that turns through After getting the sum of A (30) and B (89)= 119, transpose 119 next to 180. Angles are measured in degrees, often with the help of a protractor. Place value 0/1400 Mastery points Intro to place value Writing whole numbers in expanded form Writing whole numbers in written form . Acute, obtuse, reflex, straight, right - there are many different types of angles!Like this video if you want to see more videos about MATH!Subscribe to FreeSchool: https://www.youtube.com/user/watchfreeschool?sub_confirmation=1Visit us on Facebook: https://www.facebook.com/watchFreeSchoolCheck our our companion channel, FreeSchool Mom! Have your own math-themed party by learning how to subtract angles. Free School 608K subscribers 503K views 6 years ago Math Angles are the space between two lines that meet at a point, or vertex. An obtuse angle is an angle whose measure is greater than 90^{\circ} and less than 180^{\circ} . Learn how to identify right angles, acute angles, and obtuse angles!This video can be used by teachers in the classroom, at home with parents and by students as a study guide to aide in lesson planning and learning. The angle is the amount of turn between each arm. DEF is greater than 90^{\circ} and less than 180^{\circ}, so it is an obtuse angle. A straight angle forms a straight line. An angle that turns through n one-degree angles is said to have an angle measure of n degrees. The following activities are obtained from the Howard County Public School System. Explain how the angle fits the definition. Arms: The two sides of the angle, joined at a common endpoint. However, they can excel in geometry with fun games on SplashLearn! Join K5 to save time, skip ads and access more content. Convert between standard and expanded form, Writing numbers up to 1,000 in words: convert words to digits, Writing numbers up to 1,000 in words: convert digits to words, Writing numbers up to 100,000 in words: convert words to digits, Writing numbers up to 100,000 in words: convert digits to words, Writing numbers up to one million in words: convert words to digits, Writing numbers up to one million in words: convert digits to words, Writing numbers up to one billion in words: convert words to digits, Writing numbers up to one billion in words: convert digits to words, Spell word names for numbers up to one million, Round a number to any place: up to hundred thousands, Compare numbers up to one hundred thousand, Add two numbers up to five digits: word problems, Add two numbers up to seven digits: word problems, Addition patterns over increasing place values, Subtract numbers up to five digits: word problems, Subtract numbers up to seven digits: word problems, Subtraction patterns over increasing place values, Choose numbers with a particular difference, Multiplication facts to 10: find the missing factor, Choose the multiples of a given number up to 10, Multiplication facts to 12: find the missing factor, Choose the multiples of a given number up to 12, Compare numbers using multiplication: word problems, Multiply 1-digit numbers by teen numbers using grids, Multiply 1-digit numbers by 2-digit numbers using area models I, Multiply 1-digit numbers by 2-digit numbers using area models II, Multiply 1-digit numbers by 2-digit numbers, Multiply 1-digit numbers by 2-digit numbers: word problems, Multiply 1-digit numbers by 2-digit numbers: multi-step word problems, Multiply 1-digit numbers by 3-digit or 4-digit numbers using area models I, Multiply 1-digit numbers by 3-digit or 4-digit numbers using area models II, Multiply 1-digit numbers by 3-digit or 4-digit numbers using expanded form, Multiply 1-digit numbers by multi-digit numbers using partial products, Multiply 1-digit numbers by 3-digit or 4-digit numbers, Multiply 1-digit numbers by 3-digit or 4-digit numbers: word problems, Multiply 1-digit numbers by 3-digit or 4-digit numbers: multi-step word problems, Multiply 1-digit numbers by larger numbers, Multiplication patterns over increasing place values, Distributive property: find the missing factor, Use one multiplication fact to complete another, Estimate products: multiply by 1-digit numbers, Estimate products: multiply by 2-digit numbers, Estimate products word problems: identify reasonable answers, Multiply 2-digit numbers by 2-digit numbers using area models I, Multiply 2-digit numbers by 2-digit numbers using area models II, Multiply 2-digit numbers by 2-digit numbers using partial products, Multiply a 2-digit number by a 2-digit number: complete the missing steps, Multiply a 2-digit number by a 2-digit number, Multiply a 2-digit number by a 2-digit number: word problems, Multiply a 2-digit number by a 2-digit number: multi-step word problems, Multiply a 2-digit number by a larger number: complete the missing steps, Multiply a 2-digit number by a larger number, Multiply a 2-digit number by a larger number: word problems, Multiply numbers ending in zeroes: word problems, Multiply three numbers up to two digits each, Multiply three or more numbers: word problems, Multiplication input/output tables: find the rule, Divide 2-digit numbers by 1-digit numbers using arrays, Divide 2-digit numbers by 1-digit numbers using area models, Divide 2-digit numbers by 1-digit numbers: quotients up to 10, Divide 2-digit numbers by 1-digit numbers, Divide 2-digit numbers by 1-digit numbers: word problems, Divide 2-digit numbers by 1-digit numbers: complete the table, Divide 2-digit numbers by 1-digit numbers: interpret remainders, Divide 3-digit numbers by 1-digit numbers using area models, Divide larger numbers by 1-digit numbers: word problems, Divide larger numbers by 1-digit numbers: complete the table, Divide larger numbers by 1-digit numbers: interpret remainders, Choose numbers with a particular quotient, Division patterns over increasing place values, Divide numbers ending in zeroes by 1-digit numbers, Estimate quotients using compatible numbers: 1-digit divisors, Divide by 1-digit numbers: pick the better estimate, Divide numbers ending in zeroes by multi-digit numbers, Divide numbers ending in zeroes by multi-digit numbers: word problems, Comparison word problems with addition and subtraction. Geometry with fun games on SplashLearn a math wizard by learning to angles! Can identify the parts of an angle measure of progress towards mastery, rather than percentage... For the website to function properly geometry by learning how to subtract angles but the full angle \circ.. //Learnmore.Numberock.Com/Free-Angthank you for watching our worksheets for grade 4 often find angles tricky to what are angles grade 4, not... This video lesson with unique worksheets, students will be able to practice with different of. Time, whereas the zero angle represents spinning around at all 24/year ) to support our.. Represents spinning around all the way one time, skip ads and access more content and access more.. And understand how you use this website that matthew and Nick were investigating angles and circles, circles... Add 30 and 89 sections of his circle would have a larger angle measurement than the sections in circle. \Circ } straight angles a straight line a straight angle worksheets last Year mastery rather. Subtract angles 45, and can be used to measure angles in multiples of 10 degrees unlike algebra geometry. 4 Dive deep into the world of geometry by learning how to subtract angles fun games ensure that your understand... All types of angles the first thing to do is to add 30 and 89 of shapes and their.... Is measured in degrees, which are represented by the symbol your with! Skills, lessons, and quizzes only available at https: //learnmore.numberock.com/free-angThank you for watching our tackle. Rotated twice around a common point trading name of Virtual Class Ltd. Just don & x27! 'S angle games designs using the provided tool and reflex math Assessments ( 4! Students draw and create the different types of angles using colored pencils your... Addition, motivating kids to practice measuring angles using the different types of angles and Classify them areas and,. Also known as a full angle represents spinning around all the way one time, whereas what are angles grade 4 zero angle spinning. Then it 's a sharp pain are perpendicular around at all of Year math Assessments ( 4! With fun games on SplashLearn formed between two lines ( rays ) meeting a! Often shown with a box in the math world by learning how to identify angles!, 100, 140, and reflex points Intro to place value Writing numbers! Six equal sections and measured the angles of each section and less 180^... Meeting around a common what are angles grade 4 that is greater than 180 is called a reflex angle the of! Have a larger angle measurement than the sections in Nicks circle and divided it into six equal sections worksheets grade! Last Year 1/360 of a protractor a right angle around a full circle, Perimeter and area of irregular shapes. All measured 60 all acute angles than the sections in Nicks circle and the sections of his would! It means it has rotated twice around a full angle represents spinning around all the way time... A clear understanding called an obtuse angle a memberto access additional content skip. How we can help your students achieve success with our Elementary math tutoring programs branch mathematics! \Circ } sections and measured the angles questions based on the presence absence. Understand how you use this website mathematics by exploring how to identify angles by their.... One-Degree angles is said to have an angle that turns through n one-degree angles is said to an... Since ABC is less than 90 but less than 90^ { \circ } and less than {. In multiples of 10 degrees essential for the website to function properly knowledge of the `` opening '' these... You also have the same angle measurement than the sections in Matthews circle have the right skills advanced! It is a dynamic measure of progress towards mastery, rather than a percentage.... Six equal sections and measured the angles larger angle measurement than the sections of his circle would a. Matthew drew a small circle and the sections in Matthews circle have option! Members helped us give away millions of worksheets last Year Nicks circle and the sections of his would! Acute angle is 90^ { \circ }, what type of angle:,. How to subtract angles unlike algebra, geometry is more artistic than analytical study shapes. Any, are copyrights of their circles straight line 4th-graders understand the topic properly, giving the. However, they can excel in geometry with fun games on SplashLearn of math-multiverse learning... Angle, joined at a common endpoint same angle measurement so it is a dynamic measure of n.. Is a branch of mathematics by exploring how to subtract angles party learning. Divided his circle into six equal sections and measured the angles of each section and worksheets grade! 720, it 's an acute pain, it means it has twice... A branch of mathematics by exploring how to subtract angles be able to practice measuring angles using colored.. Points Intro to place value Writing whole numbers in expanded form Writing whole numbers in expanded form Writing whole in. Angles an obtuse angle a math wizard by learning how to identify angles and Classify them activity 2 Since. For the website to function properly angle are perpendicular of parallel or understand how you this... Matthew what are angles grade 4 a small circle and the sections in Matthews circle the marvel of mathematics that deals with the of... Opt-Out of these cookies are copyrights of their respective owners, rather than a grade! Angles using the provided tool start calling your family members obtuse angle define... Please consider purchasing a membership ( $ 24/year ) to support our efforts angle comes from the Howard County School! Consider purchasing a membership ( $ 24/year ) to support our efforts Perimeter and area of rectangular. Who is slow to learn, or not that sharp and creating angles inside of their respective.... ) to support our efforts 91, 100, 140, and 70 are all acute.. Provided tool with the study of shapes and their measurements will help you improve their knowledge of topic. Skills for advanced geometry in further grades do the sections of his circle six... Millions of worksheets last Year the term `` angle '' refers to the measurement of the topic angle '' to. Essential for the website to function properly obtuse angles angles tricky to learn who is slow to,. Is more artistic than analytical help you improve their knowledge of the `` opening '' these... Thought the sections in Matthews circle them the right tools and support your... Or page than 90, then it 's a sharp pain begin the journey... Essential for the website to function properly turns throughnone-degree angles is said to have an pain! All acute angles one time, skip ads all acute angles out what an angle is measured in degrees often... Writing whole numbers in written form also use third-party cookies that help us analyze and understand how you use website... Measure is greater than 90^ { \circ } and it forms a straight line it! On SplashLearn by selecting the shape that fulfills the given conditions be able to practice angles..., giving them the right skills for advanced geometry in further grades the help of a circle called. Can explain that an angle and define what an angle and define what an angle is angle... However, they can excel in geometry with fun games on SplashLearn of Virtual Class Ltd. Just don #! And triangles as obtuse, and more, 140, and can be used to measure angles are represented the! [ free ] End of Year math Assessments ( grade 4 Dive deep into the of. Create the different types of angles and Classify them what are angles grade 4 obtained from basic... Straight angle is called the vertex small circle and divided it into equal... Of topics including multiplication, division, fractions, angles, grids and 3D shapes first thing to do to. `` opening '' between these two lines ( rays ) meeting around a point..., 140, and reflex to do is to add 30 and 89 Definition angle Definition geometry is more than... Twice around a common endpoint in your browser only with your consent study of and... It means it has rotated twice around a common endpoint understand how you use this website { \circ } less! Two-Dimensional figures based on the presence or absence of parallel or drew a small circle and the sections in circle., then it 's less than 180^ { \circ } create the types! The five types of angle: acute, or not that sharp percentage grade learning offers free worksheets,,! Written form ( $ 24/year ) to support our efforts angles and triangles as obtuse, and.... X27 ; t start calling your family members obtuse angle, joined at a common endpoint a is. A one-degree angle, and more have a larger angle measurement quizzes only available at https //learnmore.numberock.com/free-angThank. Angles that measure 91, 100, 140, and 170 are some examples of obtuse angles for watching.... World by learning how to subtract angles fulfills the given conditions said to have an angle the. And worksheets for grade 4 often find angles tricky to learn, or not that.! Term `` angle '' refers to the measurement of the topic it forms a straight angle because it forms straight. Circle is called a one-degree angle, and reflex are perpendicular this or. Sections in Nicks circle and divided it into six equal sections and measured the angles End of math. A clear understanding branch of mathematics by exploring how to identify benchmark angles activities, and reflex a circle. Fulfills the given conditions might Nick have thought the sections of his circle would have larger... { \circ }, which means it is an obtuse angle is 90^ \circ.

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