In the given figure we can determine the coordinate of point M from the graph, we get:

We can also determine the coordinates of point N as:

Now, to determine the length of segment MN, we need to subtract the x-coordinate of M from the coordinates of N, we get:

Subtracting the fractions we get:

Now, to obtain the length of AB we need to subtract the x-coordinate of A from the x-coordinate of B.
The coordinates of A are determined from the graph:

The coordinates of B are:

Therefore, the length of segment AB is:

Now we do the same procedure to determine the segment of CD. The coordinates of C are:

The coordinates of D are:

Therefore, CD is:

Now, we determine MN as half the sum of the bases. The bases are AB and CD, therefore:

Therefore, we have proven that the median of a trapezoid equals half the sum of its bases.