To prove the Pythagorean Theorem based on similarity of triangles, you can always use the 2nd construction.
The 2nd construction is achieved by drawing a perpendicular line joining between the right angle of the triangle and the hypothesis (as shown in the attachment).
Note: The Pythagorean Theorm is stated as follows: (length of hypotenuse)^2 = (length of side1)^2 + (length of side2)^2
