Bài tập 5.68 trang 209 SBT Toán 11

\(\begin{array}{l}
 \Rightarrow y\prime  = \frac{{(\cos x - \cos x + x\sin x)(\cos x + x\sin x) - ( - \sin x + \sin x + x\cos x)(\sin x - x\cos x)}}{{{{(\cos x + x\sin x)}^2}}}\\
 = \frac{{x\sin x(\cos x + x\sin x) - x\cos x(\sin x - x\cos x)}}{{{{(\cos x + x\sin x)}^2}}}\\
 = \frac{{{x^2}\sin 2x + {x^2}\cos 2x}}{{{{(\cos x + x\sin x)}^2}}} = \frac{{{x^2}}}{{{{(\cos x + x\sin x)}^2}}}
\end{array}\)