Exercício sobre trigonometria

Resolver as equações:

$\phantom{X}cos\,3x\,-\,cos\,x\,=\,0\phantom{X}$

$\phantom{X}cos\,5x\,=\,cos\left(x\,+\,\dfrac{\,\pi\,}{\,3\,}\right)\phantom{X}$

resposta: a) $\,S\,=\,\lbrace\,x\,\in\,{\rm\,I\!R}\;|\;\,x\,=\,k\pi\;ou\;x\,=\,\,\frac{k\pi}{2}\,\rbrace\,$
b) $\,S\,=\,\lbrace\,x\,\in\,{\rm\,I\!R}\;|\;\,x\,=\,\frac{\pi}{12} + \frac{k\pi}{2} \; ou\; x\,=\frac{-\pi}{18}\,+\,\frac{k\pi}{3} \rbrace\,$

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