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For $$k \in \mathbb{R}$$, let the solutions of the equation $$\cos\left(\sin^{-1}\left(x \cot\left(\tan^{-1}\left(\cos(\sin^{-1} x)\right)\right)\right)\right) = k$$, $$0 < |x| < \frac{1}{\sqrt{2}}$$ be $$\alpha$$ and $$\beta$$, where the inverse trigonometric functions take only principal values. If the solutions of the equation $$x^2 - bx - 5 = 0$$ are $$\frac{1}{\alpha^2} + \frac{1}{\beta^2}$$ and $$\frac{\alpha}{\beta}$$, then $$\frac{b}{k^2}$$ is equal to
Correct Answer: 12
Using
$$\cos(\sin^{-1}x)=\sqrt{1-x^2},$$
we get
$$\tan^{-1}\left(\cos(\sin^{-1}x)\right)=\tan^{-1}\left(\sqrt{1-x^2}\right).$$
Hence,
$$\cot\left(\tan^{-1}\left(\sqrt{1-x^2}\right)\right)=\frac{1}{\sqrt{1-x^2}}.$$
Therefore,
$$\cos\left(\sin^{-1}\left(\frac{x}{\sqrt{1-x^2}}\right)\right)=k.$$
Using
$$\cos(\sin^{-1}t)=\sqrt{1-t^2},$$
we obtain
$$\sqrt{1-\frac{x^2}{1-x^2}}=k.$$
$$\sqrt{\frac{1-2x^2}{1-x^2}}=k.$$
Squaring both sides,
$$k^2=\frac{1-2x^2}{1-x^2}.$$
Hence,
$$x^2=\frac{1-k^2}{2-k^2}.$$
Since
$$0<|x|<\frac{1}{\sqrt2},$$
the two solutions are
$$\alpha=\sqrt{\frac{1-k^2}{2-k^2}},\qquad\beta=-\sqrt{\frac{1-k^2}{2-k^2}}.$$
Therefore,
$$\frac{\alpha}{\beta}=-1.$$
Also,
$$\frac{1}{\alpha^2}+\frac{1}{\beta^2} =\frac{2}{\alpha^2} =2\cdot\frac{2-k^2}{1-k^2}.$$
The roots of
$$x^2-bx-5=0$$
are
$$2\cdot\frac{2-k^2}{1-k^2}\quad\text{and}\quad-1.$$
Using the product of roots,
$$-2\cdot\frac{2-k^2}{1-k^2}=-5.$$
Hence,
$$2(2-k^2)=5(1-k^2).$$
$$4-2k^2=5-5k^2.$$
$$3k^2=1.$$
$$k^2=\frac13.$$
Now, using the sum of the roots,
$$b=2\cdot\frac{2-k^2}{1-k^2}-1.$$
Substituting
$$k^2=\frac13,$$
$$b=2\cdot\frac{\frac53}{\frac23}-1.$$
$$=5-1.$$
$$=4.$$
Therefore,
$$\frac{b}{k^2}=\frac{4}{\frac13}=12.$$
Hence,
$$\boxed{12}$$
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