cos(x-7π/6)=0
no intervalo 0<y<2π
cosy=π/2 ou cosy=3π/2
logo,
x-7π/6=π/2
x=π/2+7π/6
x=(3π+7π)/6
x=10π/6
x=5π/3
ou
x-7π/6=3π/2
x=3π/2+7π/6
x=(9π+7π)/6
x=16π/6
x=8π/3
