jika tan2a= t maka cos4a=...

Tan 2a = t
sin A = t/√(t² + 1)
cos A = 1/√(t² + 1)

cos 4A = 1 - 2 sin² 2A
= 1 - 2 (2 sin A cos A)²
= 1 - 2 (4 sin² A cos² A)
= 1 - 8 sin² A cos² A
= 1 - 8 {t/√(t² + 1)}² {1/√(t² + 1)}²
= 1 - 8 {t²/(t² + 1)} {1/(t² + 1)}
= 1 - 8t²/(t^4 + 2t² + 1)
= {(t^4 + 2t² + 1)/(t^4 + 2t² + 1)} - {8t²/(t^4 + 2t² + 1)}
= t^4 - 6t² + 1)/(t^4 + 2t² + 1)