Grade 3Math
AnglesCompare their size by putting them on top of each other
The shape made by 2 sides coming out from one vertex is called an angle. You can compare the size of angles by putting them on top of each other. We fold isosceles triangles and equilateral triangles to check that their angles are the same size.
The narration and the pictures are in Japanese. You can hear the same words that are used in class at a Japanese school.
- Video
- 1:28（6 scenes）
- Problems
- 3 problems
Let's try
Write the math sentence in your notebook first, then open the answer.
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1There is an isosceles triangle where the 2 sides coming out from vertex カ are the same length. Which two angles are the same size?
Hint
If you fold it so the sides of equal length are on top of each other, which angle meets which angle?
Answer
Angle キ and angle ク. If you fold it so the sides of equal length are on top of each other, these two angles fit exactly on top of each other.
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2You put angle あ and angle い on top of each other, with the vertex and one side lined up. The other side of angle い stuck out. Which angle is bigger?
Hint
Does the side stick out on the bigger angle or the smaller angle?
Answer
Angle い. When you line them up and put them on top of each other, the one whose other side sticks out is the bigger angle.
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3Which triangle has 3 angles that are all the same size: an isosceles triangle or an equilateral triangle?
Hint
Which triangle has all 3 sides the same length?
Answer
An equilateral triangle. Even when you fold it a different way, the angles fit exactly on top of each other.
Words in this lesson
- angle 角
- The shape made by 2 sides coming out from one vertex.
- size of an angle 角の 大きさ
- A size that you can compare by putting angles on top of each other. Angles that fit exactly on top of each other are the same size.
- vertex 頂点
- The corner point where two sides meet.
For parents and guardians
- The names of lessons and the order of topics differ slightly between textbooks. This page follows the content of Japan's national curriculum guidelines (the Course of Study).
- In 3rd grade, the size of an angle is not shown with a number; angles are compared by putting them on top of each other. Try cutting paper into an isosceles triangle or an equilateral triangle, folding it so that the sides of equal length are on top of each other, and checking together that the angles fit exactly.
- Thinking of the size of an angle as how wide the sides are open (the amount of turning), the unit of degrees, and how to use a protractor are learned in 4th grade. The angle marks on the screen (colored fan shapes) show where the angles are; the size of the mark is not the size of the angle.