I got this question in a complex numbers test and I could not figure it out. Any help would be appreciated.

Question:

R is a positive number and $\displaystyle z_1$ and $\displaystyle z_2$ are complex numbers. Show that the points which represent respectively the numbers $\displaystyle z_1 $, $\displaystyle z_2$, $\displaystyle \frac{z_1-iRz_2}{1-iR}$ form the vertices of a right angled triangle