Prove the midpoint formula by using the following procedure.
a. Show that the distance between $\left(x_1, y_1\right)$ and $\left(\frac{x_1+x_2}{2}, \frac{y_1+y_2}{2}\right)$ is equal to the distance between $\left(x_2, y_2\right)$ and $\left(\frac{x_1+x_2}{2}, \frac{y_1+y_2}{2}\right)$.
b. Use the procedure from Exercise 89 and the distances from part (a) to show that the points $\left(x_1, y_1\right)$, $\left(\frac{x_1+x_2}{2}, \frac{y_1+y_2}{2}\right)$, and $\left(x_2, y_2\right)$ are collinear.