Solving Angles in a Triangle

About this video

How to find unknown angles in a triangle using the fact that the three interior angles always add up to 180°.

What it covers

• Interior angles of a triangle sum to 180°
• Isosceles triangles have two equal base angles
• Setting up an equation to solve for x

Exercises to try

Work each one out, then click to reveal the answer.

1. 1

   A triangle has angles 90°, 35° and x. Find x.

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   180° − 90° − 35° = 55°
2. 2

   An isosceles triangle has a 40° apex angle. Find each base angle.

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   (180° − 40°) ÷ 2 = 70°
3. 3

   Angles are x, 2x and 3x. Find x.

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   x + 2x + 3x = 180° → 6x = 180° → x = 30°