Resposta :
cos(a+b)=cosa.cosb-sena.senb
cos(π+x)=cos(π).cos(x)-sen(π).sen(x)
cos(π+x)=-1.cos(x)-0.sen(x)
cos(π+x)=-cos(x)
cos (
cos (π + x) = -cos x
cos π + cos x= - cos x
-1 + cos x = -cos x (cosseno de pi = -1 )
-1 +cos x + cos x = 0
2cos x -1 = 0
2cosx = 1
cosx = 1/2