Resposta:
[tex]x=\frac{3\pi }{2}[/tex]
Explicação passo a passo:
cos2x + 4cosx + 3 = 0, [0, π]
2cos²x - 1 + 4cosx + 3 = 0
2cos2x + 4cosx + 2 = 0
cos2x + 2cosx + 1 = 0
[tex]\Delta=2^2-4.1.1\\\\\Delta=4-=4=0\\\\cosx=\frac{-2}{2}=-1\\\\cosx=cos\frac{3\pi}{2} \\\\x=\frac{3\pi}{2}[/tex]
