sin^2 A . cos^ A adalah

cos(a-b) = cos(a) cos(b) + sin(a) sin(b)

jika b = a, maka:
cos(a-a) = cos(a) cos(a) + sin(a) sin(a)
cos(0) = cos²a + sin²a
1 = cos²a + sin²a

jadi, sin²a + cos²a = 1