JEE Advance - Mathematics (2008 - Paper 2 Offline - No. 7)
$$x\sqrt {{x^2} - 1} \,\,dy - y\sqrt {{y^2} - 1} \,dx = 0$$ satify $$y\left( 2 \right) = {2 \over {\sqrt 3 }}.$$
STATEMENT-1 : $$y\left( x \right) = \sec \left( {{{\sec }^{ - 1}}x - {\pi \over 6}} \right)$$ and
STATEMENT-2 : $$y\left( x \right)$$ given by $${1 \over y} = {{2\sqrt 3 } \over x} - \sqrt {1 - {1 \over {{x^2}}}} $$
Explanation
The given differential equation is
$$x\sqrt {{x^2} - 1} dy - y\sqrt {{y^2} - 1} dx = 0$$
$$ = \int {{{dy} \over {y\sqrt {{y^2} - 1} }} = \int {{{dx} \over {x\sqrt {{x^2} - 1} }}} } $$
$$ \Rightarrow {\sec ^{ - 1}}y = {\sec ^{ - 1}}x + c$$
$$y = \sec [{\sec ^{ - 1}}x + C]$$ [$$\because$$ $$y(2) = {2 \over {\sqrt 3 }}$$]
$${2 \over {\sqrt 3 }} = \sec ({\sec ^{ - 1}}2 + C)$$
$$ = {\sec ^{ - 1}}{2 \over {\sqrt 3 }} - {\sec ^{ - 1}}2 = C$$
$$C = {\pi \over 6} - {\pi \over 3} = {{ - \pi } \over 6}$$ [$$\therefore$$ $$y = \sec \left[ {{{\sec }^{ - 1}}x - {\pi \over 6}} \right]$$]
Statement 1 is true
Also, $${1 \over y} = \cos \left[ {\cos - {\pi \over x} - {\pi \over 6}} \right]$$
$$\cos \left( {{{\cos }^{ - 1}}{1 \over x}} \right)\cos {\pi \over 6} + \sin ({\cos ^{ - 1}}{1 \over x})\sin {\pi \over 6}$$
$$ = {1 \over y} = {{\sqrt 3 } \over {2x}} + {1 \over 2}\sqrt {1 - {1 \over {{x^2}}}} $$
$$\therefore$$ Statement 2 is false.
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