Limits, Continuity and Differentiability
79935
If \(\lim _{x \rightarrow \frac{\pi}{4}} \frac{4 \sqrt{2}-(\cos x+\sin x)^{5}}{1-\sin 2 x}\), then \(x\) is equal to
1 \(\sqrt{2}\)
2 \(3 \sqrt{2}\end{array}\)
3 \(5 \sqrt{2}\)
4 None of the above
Explanation:
(C) : \(\lim _{x \rightarrow \frac{\pi}{4}} \frac{4 \sqrt{2}-(\cos x+\sin x)^{5}}{1-\sin 2 x}\)
\(=\lim _{x \rightarrow \frac{\pi}{4}} \frac{4 \sqrt{2}-(\cos x+\sin x)^{5}}{(\sin x-\cos x)^{2}}\)
Apply L-Hospital rule,
\(=\lim _{x \rightarrow \frac{\pi}{2}} \frac{0-5(\cos x+\sin x)^{4} \cdot(-\sin x+\cos x)}{2(\sin x-\cos x) \cdot(\cos x+\sin x)}\)
\(=\lim _{x \rightarrow \frac{\pi}{4}} \frac{5(\cos x+\sin x)^{4} \cdot(\sin x-\cos x)}{2(\sin x-\cos x) \cdot(\cos x+\sin x)}=\lim _{x \rightarrow \frac{\pi}{4}} \frac{5}{2}(\cos x+\sin x)^{3}\)
\(=\frac{5}{2}\left(\frac{1}{\sqrt{2}}+\frac{1}{\sqrt{2}}\right)^{3}=\frac{5}{2}\left(\frac{2}{\sqrt{2}}\right)^{3}=\frac{5}{2} \times(\sqrt{2})^{3}=5 \sqrt{2}\)