Réponses

2013-10-16T23:38:40+02:00
On a cos(2t)= 2(cos(t))^2 - 1. Posons X = cos(2t), donc cos(4t)=cos(2(2t))=2 X^2 - 1.
On a X^2 = (2(cos(t))^2 - 1)^2 = 4(cos(t))^4 - 4(cos(t))^2 +1), donc cos(4t)=2 X^2 - 1 = 8(cos(t))^4 - 8(cos(t)^2) +1