We can start with the formula for the area of a triangle.

In our case:

So we can have:

We also know that:

If we look at our smaller triangles, we see that:

So another way to calculate area is:

We recall:

Then we divid by AD*BC:

Since:

We can substitute our denominators:

Therefore, we arrive at:

Now let's prove:

We proved above that the follow is true:

Now let's look at our triangle.

We can easily see that:

Or

So by a little substitution we get the following: