1.
\(y=1+cosx.sin\left(\pi+\frac{\pi}{2}-2x\right)=1-cosx.cos2x\)
\(y\left(-x\right)=1-cos\left(-x\right).cos\left(-2x\right)=1-cosx.cos2x=y\left(x\right)\)
Hàm chẵn
2.
\(1+2cosx\ge0\Leftrightarrow cosx\ge-\frac{1}{2}\)
\(\Leftrightarrow-\frac{2\pi}{3}+k2\pi\le x\le\frac{2\pi}{3}+k2\pi\)