Objectives
To use parallel lines to prove a theorem about triangles
To find measures of angles of triangles
Triangle Theorems Activity
In the Solve It, you may have discovered that you can rearrange the corners of the triangle to form a straight angle. You can do this for any triangle.
Essential Understanding The sum of the angle measures of a triangle is always the same.
The Solve It suggests an important theorem about triangles. To prove this theorem, you will need to use parallel lines.
Through a point not on a line, there is one and only one line parallel to the given line.
There is exactly one line through P parallel to l.