Consider a right angled triangle with hypotenuse c and legs a and b. Let sin theta = a/c .
Then cos theta = b/c and tan (90-theta) = b/a
Then 2 sin theta cos theta / tan (90 - theta)
= 2 * a/c * b/c / b/a
= 2 * ab/c^2 * a/b
= 2 a^2 / c^2 = 2 sin^2 theta