Answer:
Rearrange the equations that result from use of the Pythagorean theorem.
Explanation:
Transversal AB crossing parallel lines AD and BC makes supplementary interior same-side angles A and B. Since A = 90°, B must be 90°. The Pythagorean theorem then applies in the right triangles ABC and ABD.
We can use that theorem to write two expressions for AB^2:
BD^2 -AD^2 = AB^2 = AC^2 -BC^2
The middle expression, AB^2, isn't needed beyond this point. Adding (AD^2 -AC^2) to both sides of the equation gives the desired result:
BD^2 -AC^2 = AD^2 -BC^2