Buktikan bahwa cos²a+sin²a=1

cos a = x/r
sin a= y/r


dan x^2 + y^2 = r^2
masing-masing bagi r^2
x^2/r^2 + y^2/r^2 = r^2/r^2
(x/r)^2 + (y/r)^2 = 1
cos^2a + sin^2a = 1