résoudre
cos(3x) = -8 cos(x)
4cos(x)³-3cos(x)= -8cos(x)
4cos (x)³-3cos(x)+8cos(x)=0
4cos(x)³+5cos(x)=0
cos(x)×(4cos(x²) +5)=0
cos(x)=0
4cos(x)²+5=0
x= π/2+kπ , k € Z
