Triangles Contain 180°

Section A: Unit Notes

In a triangle, the three interior angles always add to 180°:

A + B + C = 180°

Proof

This is a proof that the angles in a triangle equal 180°:

The top line (that touches the top of the triangle) runs parallel to the triangle's base.

Notice that:

• angles A are the same
• angles B are the same

And you can see that A + C + B rotates from one side of the straight line to the other, or a full 180°

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Section B: Video Lesson

To better understand triangles, contain 180^o…click the video below.

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Section C: Online Module

180 degrees in a triangle| School Yourself Geometry