Grade 4 (age 9-10)

Types of Triangles

A triangle is a shape made of three sides and three angles. You can spot them everywhere, like road signs and hangers. By side length they are equilateral, isosceles, or scalene; by angle they are acute, right, or obtuse. Their angles always add up to 180 degrees, and here you tap the buttons to compare how sides and angles differ.

Types of Triangles

🔺 Two Ways to Classify Triangles

①By side length: equilateral, isosceles, scalene

②By angle size: acute, right, obtuse

③A triangle can belong to both classifications at once!

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Classification by Side Length

📏 By Side Length

①Equilateral: all three sides equal (all angles also 60°)

②Isosceles: two sides equal (the two angles opposite are also equal)

③Scalene: all three sides different

Summary

Equilateral Triangle

3 equal sides, 3 angles = 60°

The most balanced triangle

Right Triangle

One angle = 90° (right angle)

A triangle with an L-shaped corner

Sum of Interior Angles

Sum of 3 angles = 180°

Any triangle's angles always sum to 180°

🎯 Key Points

①By sides: equilateral(3 equal), isosceles(2 equal), scalene(all different)

②By angles: acute(all<90°), right(one=90°), obtuse(one>90°)

③Sum of angles is always 180°

④Equilateral also counts as isosceles

⑤In a right triangle, the side opposite the right angle is the longest (hypotenuse)

Examples and Unit Test

Example 1

What is a triangle with three equal sides called, and what is the size of each angle?

A triangle with three equal sides is equilateral. Its three angles are also all equal.

Dividing the angle sum 180° equally into three: 180 ÷ 3 = 60°.

▸ Equilateral, each angle 60°

An equilateral triangle has three equal sides and angles; each angle is 60°.

Example 2

What is a triangle with exactly two equal sides called?

Classify triangles by side length.

Two equal sides make an isosceles triangle. (Three equal sides = equilateral.)

▸ Isosceles

An equilateral triangle also counts as isosceles (it meets the two-equal-sides condition).

Unit Test

What is a triangle with one 90° angle called?

①acute

②right

③obtuse

④equilateral

▸ ② right

Classify triangles by angle size.

A triangle with one right angle (90°) is a right triangle.

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Angles

Quadrilaterals