3．（a）Solve，for \(0 \leq x < 2 \pi\) ， $$\cos x + \cos 2 x = 0$$ （b）Find the exact value of \(x , x \geq 0\) ，for which $$\arccos x + \arccos 2 x = \frac { \pi } { 2 }$$ ［ \(\arccos x\) is an alternative notation for \(\cos ^ { - 1 } x\) ．］