Problem $3$

For which $x$ is the function $f(x)=cos(4x+17)$ decreasing?

$($A$)x$ in $(\frac{-17}{4},)$

$x$ in $(2k,2(k+1))$ for some integer $k$

$($C$)x$ in $(\frac{k}{2}+\frac{-17}{4},\frac{(k+1)}{2}+\frac{-17}{4})$ for some integer $k$

$($D$)x$ in $(\frac{(k+1)}{2},\frac{(k+2)}{2})$ for some integer $k$

$($E$)x$ in $(\frac{(k+1)}{2}+\frac{-17}{4},\frac{(k+2)}{2}+\frac{-17}{4})$ for some integer $k$