Limits, Continuity and Differentiability
80307
If \(y=\cos ^{2}\left(\frac{5 x}{2}\right)-\sin ^{2}\left(\frac{5 x}{2}\right)\), then \(\left(\frac{d^{2} y}{d x^{2}}\right)=\)
1 \(5 \sqrt{1-y^{2}}\)
2 \(25 y\)
3 \(-25 y\)
4 \(-5 \sqrt{1-y^{2}}\)
[MНT CET-2020]
Explanation:
(C) : Given,
\(y=\cos ^{2}\left(\frac{5 x}{2}\right)-\sin ^{2}\left(\frac{5 x}{2}\right)\)
We know that,
So,
\(\cos ^{2} x-\sin ^{2} x=\cos 2 x\)
\(y=\cos 2\left(\frac{5 x}{2}\right)\)
\(y=\cos 5 x\)
Now, differentiate with respect to \(x\)
\(\frac{d y}{d x}=-5 \sin 5 x\)
\(\therefore \quad \frac{\mathrm{d}^{2} \mathrm{y}}{\mathrm{dx}^{2}}=-25 \cos 5 \mathrm{x}\)
On putting the value \((\cos 5 x=y)\) in above equation. Then,
\(\frac{d^{2} y}{d x^{2}}=-25 y\)