b) `2sinx+\sqrt2=0`
`<=> sinx=-\sqrt2 /2`
`<=>` \(\left[ \begin{array}{l}x=\dfrac{π}{4}+k2π\\x=π+\dfrac{π}{4}+k2π\end{array} \right.\)
`<=>` \(\left[ \begin{array}{l}x=\dfrac{-π}{4}+k2π\\x=\dfrac{5π}{4}+k2π\end{array} \right.\) `(k \in \mathbbZ)`
f) `2cos2x+1=0`
`<=> cos2x=-1/2`
`<=>` \(\left[ \begin{array}{l}2x=\dfrac{2π}{3}+k2π\\2x=\dfrac{-2π}{3}+k2π\end{array} \right.\)
`<=>` \(\left[ \begin{array}{l}x=\dfrac{π}{3}+kπ\\x=\dfrac{-π}{3}+kπ\end{array} \right.\)
`<=> x=π/3+kπ/2 (k \in \mathbbZ)`
