If $P_{1}=\left(x_{1}, y_{1}\right), P_{2}=\left(x_{2}, y_{2}\right)$ and $M=\left(\frac{x_{1}+x_{2}}{2}, \frac{y_{1}+y_{2}}{2}\right)$,
show that $d\left(P_{1}, M\right)=d\left(M, P_{2}\right)=\frac{1}{2} d\left(P_{1}, P_{2}\right) .$ (This is one step in the proof of Theorem 2.)