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Differential Calculus> cbse 2013 Maths foreign...

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please sole the following problem

prove that the differential eqn x^2dy/dx-xy=1+cos(x/y) , xnot=0 is homogeneous. find the particular solution of tis differential eqn given that when x=1 y=pi/2

Rhinithaa PT , 13 Years ago

Grade 11

1 Answers

Jitender Singh

Ans:

$x^{2}\frac{dy}{dx}-xy=1+cos(\frac{x}{y})$

Let x = uy

$1 = u\frac{dy}{dx}+y\frac{du}{dy}$

$\frac{dy}{dx}=\frac{1-y\frac{du}{dy}}{u}$

$(uy)^{2}\frac{1-y\frac{du}{dy}}{u}-uy^{2}=1+cosu$

$-uy^{3}\frac{du}{dy}=1+cosu$

$\int \frac{u}{1+cosu}du=\int \frac{-1}{y^{3}}dy = \frac{1}{2y^{2}}+c$

$\int \frac{u}{1+cosu}du=\int \frac{u}{2cos^{2}\frac{u}{2}}du=\frac{1}{2}\int u.sec^{2}\frac{u}{2}du$

Integration by parts

$=\frac{1}{2}[2utan\frac{u}{2}-\int 2tan\frac{u}{2}du] = utan\frac{u}{2}-2lnsec\frac{u}{2}$

$utan\frac{u}{2}-2lnsec\frac{u}{2} = \frac{1}{2y^{2}}+c$

$\frac{x}{y}tan\frac{x}{2y}-2lnsec\frac{x}{2y} = \frac{1}{2y^{2}}+c$

$x = \frac{\pi }{2}\rightarrow y = 1$

$\frac{\pi }{2}tan\frac{\pi }{4}-2lnsec\frac{\pi }{4} = \frac{1}{2}+c$

$c = \frac{\pi }{2}-2lnsec\sqrt{2} - \frac{1}{2}$

$c = \frac{\pi -1-ln4}{2}$

$\frac{x}{y}tan\frac{x}{2y}-2lnsec\frac{x}{2y} = \frac{1}{2y^{2}}+\frac{\pi -1-ln4}{2}$

Thanks & Regards

Jitender Singh

IIT Delhi

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