Area of a Triangle

In the figure, the vertices of $△ABC$ are $A(0, 6), B(8, 0) \mathrm{and} \mathrm{C}(5, 8)$. If $CD\perp AB$, then find the length of altitude $CD$.

The vertices of a $∆ABC$ are $A(-5, -1), B(3, -5), C(5, 2).$ Show that the area of the $∆ABC$ is four times the area of the triangle formed by joining the mid-points of the sides of the triangle $ABC.$