If two pairs of corresponding angles in a pair of triangles are congruent, then the triangles are similar. We know this because if two angle pairs are the same, then the third pair must also be equal. When the three angle pairs are all equal, the three pairs of sides must also be in proportion.
Answer: To determine similarity, find the missing measure using the interior angle sum of a triangle. Then compare corresponding angles. If two pairs of angles are congruent, the triangles are similar because of the angle-angle criterion.
