[header] How to prove similar triangles [title] Define the angle-angle (aa) theorem. [step] Two triangles can be proved similar by the angle-angle theorem which states: if two triangles have two congruent angles, then those triangles are similar. [substeps] This theorem is also called the angle-angle-angle (aaa) theorem because if two angles of the triangle are congruent, the third angle must also be congruent. 
This is because the angles of a triangle must sum to 180 °. [title] Identify the measure of at least two angles in one of the triangles.