Grade 4 (age 9-10)

grade 4 Types of Triangles

Types of Triangles

A triangle has three sides and three angles. Road signs and hangers are everyday places to spot them. By side length they are equilateral, isosceles, or scalene; by angle they are acute, right, or obtuse. The three angles always add up to 180 degrees; tap the buttons and change the size to compare sides and angles.

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!

Compare Directly

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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

④By angles too. An acute triangle needs all three angles less than 90°. One acute angle is not enough

⑤The size slider only zooms. It does not change the type

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

Types of Quadrilaterals