geometry · Year 7 · Suggested learning sequence
Angles in a triangle
The three angles inside any triangle add up to 180°. If you tear off the corners of a paper triangle, they fit together on a straight line. To find a missing angle, add the two you know and take the total from 180. A small square means 90°. In an isosceles triangle, two sides are equal, so the two angles at the ends of the third side are equal too.
Sample questions
Find the missing angle in the triangle, marked with a question mark. The diagram is not drawn to scale.
- The angles in a triangle add up to 180°.
- Add the angles you know: 60 + 85 = 145.
- 180 − 145 = 35, so the missing angle is 35°.
Answer: 35
Find the missing angle in the triangle, marked with a question mark. The diagram is not drawn to scale.
- The angles in a triangle add up to 180°.
- Add the angles you know: 35 + 65 = 100.
- 180 − 100 = 80, so the missing angle is 80°.
Answer: 80
Find the missing angle in the triangle, marked with a question mark. The diagram is not drawn to scale.
- The angles in a triangle add up to 180°.
- Add the angles you know: 35 + 105 = 140.
- 180 − 140 = 40, so the missing angle is 40°.
Answer: 40