9.
Tham khảo:
cos3x + sin3x = cosx + sin2x + sin x giải giúp e vs ạ - Hoc24
10.
ĐK: \(x\ne\dfrac{\pi}{2}+k\pi\)
\(pt\Leftrightarrow\left(2cosx-3\right)^2+\left(3tanx+1\right)^2=0\)
\(\Leftrightarrow\left\{{}\begin{matrix}2cosx-3=0\\3tanx+1=0\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}cosx=\dfrac{3}{2}\left(vn\right)\\tanx=-\dfrac{1}{3}\end{matrix}\right.\)
\(tanx=-\dfrac{1}{3}\Leftrightarrow x=arctan\left(-\dfrac{1}{3}\right)+k\pi\)
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