Angles on a straight line add up to 180°. Angles around a point add up to 360°. The angles in a triangle add up to 180°. Vertically opposite angles, made where two lines cross, are equal.
To find a missing angle, write the fact you are using, add the angles you know and subtract from the total. Angles 123°, 102° and x meet at a point: 123 + 102 = 225, and 360 − 225 = 135°. In an isosceles triangle, the two base angles are equal.
Worked example
A triangle has angles of 48° and 65°. Find the third angle.
- Angles in a triangle add to 180°.
- 48 + 65 = 113 and 180 − 113 = 67.
Answer:67°
Key words
- vertically opposite
- equal angles across a crossing
- isosceles
- a triangle with two equal sides and angles
- angles at a point
- add to 360°
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Common Core-referenced: 7.G.B.5