One way to measure indirectly is to use similar triangles. In similar figures, corresponding angles are congruent, and corresponding sides (or segments) are in proportion. In similar triangles, however, one or the other will suffice. In other words, if the corresponding sides alone are in proportion, the triangles must be similar.
These triangles have proportional corresponding sides (a ratio of 1:2):
Likewise, if the corresponding angles alone are congruent, the triangles must be similar. Notice that because the sum of angles in a triangle must be 180 degrees, we only need to know that two of the corresponding angles are congruent to know that they are similar. Note 2
These triangles have congruent corresponding angles: