In the past, you may have written and solved equations with variables on both sides of the equation. Some situations in Geometry lend themselves to writing and solving equations. In this lesson, you will use informal arguments to establish some geometric facts. Then, you will use those facts to set up and solve equations.

A pipeline that crosses two parallel roads creates special angle relationships.

Street patterns, such as this one in Jerusalem, create triangles that also have special angle relationships.

Source: *Jerusalem triangle.jpg, Orrling, Wikimedia Commons*

There are also special relationships found in similar triangles.