In a rhombus, the diagonals intersect at an angle of 90º.
Let O be the intersection of both diagonals.
Since the diagonals on a parallelogram intersect at the middle point, and all the sides of a rhombus have the same meaure, then all the triangles that are formed (ROS, TOS, TOQ, ROQ) are congruent.
Then, we know that:
Since the angles 3 and 2 are complementary angles, then:
Since the angles 4 and 3 are congruent, then:
Therefore: