Finding the Third Angle

Part of the Angles section of Coddy's Geometry journey. Lesson 40 of 59.

With two angles of a triangle known, the third is what is left of 180°. Add the two you know and take the total from 180.

$A triangle with angles 50, 70 and 60 degrees$

Here the angles at A and B are 50° and 70°, together 120°. The angle at C is 180 - 120 = 60°. Check: 50 + 70 + 60 = 180.

The size of the drawing does not matter. A triangle drawn with a big angle at C still has whatever 180 minus the other two says.

Challenge

Two angles of triangle ABC are labelled. Find the angle at C, then press Check.

Try it yourself

$Find the marked part of the figure$

Find the marked part of the figure

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

All lessons in Angles

1Meet the Angle

4Vertical Angles

7Parallels and Transversals

10Isosceles and Equilateral

2Naming and Equations

5Around a Point

8Angle Chases

11The Exterior Angle

3Right Angles and Complements

6The Angle Bisector

9Triangle Angle Sum

12Final Challenges