Read the proof. What is step 2?

sin(2x) = cos(x)cos(x) – sin(x)sin(x)
sin(2x) = sin(x)cos(x) – sin(x)cos(x)
sin(2x) = cos(x)cos(x) + sin(x)sin(x)
sin(2x) = sin(x)cos(x) + sin(x)cos(x)

Here are the coordinates for eight points on the coordinate plane (q > p > 0): a (0, 0), b (p, 0), c (q, 0), d (p + q, 0), e (0, q), f (p, q), g (q, q), and h (p + q, q). which four points, if any, are on the vertices of a square?