3.
\(cosx=t\in\left[-1;1\right]\Rightarrow y=f\left(t\right)=-t^2-2t+1\)
\(-\dfrac{b}{2a}=-1\)
\(f\left(-1\right)=2\) ; \(f\left(1\right)=-2\)
\(\Rightarrow-2\le y\le2\Rightarrow\) có 5 giá trị nguyên của T
4.
\(cos^2x-cosx-2=0\Rightarrow\left[{}\begin{matrix}cosx=-1\\cosx=2\left(loại\right)\end{matrix}\right.\)
\(\Rightarrow x=\pi+k2\pi\)
\(\Rightarrow x=\left\{\pi;3\pi\right\}\Rightarrow\pi.3\pi=3\pi^2\)