Answer
The two triangles (with $d_{o}$ and $d_{i}$ as bases) are similar to each other, because all angles of one triangle are same as those of the other triangle due to the law of reflection. Furthermore, the triangles share same height, the common vertical side. Therefore, the two triangles are identical.
Hence, $d_{0}=d_{i}$
Work Step by Step
The two triangles (with $d_{o}$ and $d_{i}$ as bases) are similar to each other, because all angles of one triangle are same as those of the other triangle due to the law of reflection. Furthermore, the triangles share same height, the common vertical side. Therefore, the two triangles are identical.
Hence, $d_{0}=d_{i}$
