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CBSE Class 12 Math 2009 Solved Paper

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Question : 20 of 29
Marks: +1, -0
Find the particular solution, satisfying the given condition, for the following differential equation:
dydxyx\frac{dy}{dx} - \frac{y}{x} + cosec (yx)\left(\frac{y}{x}\right) = 0 , y = 0 when x = 1
Solution:  
dydxyx\frac{dy}{dx} - \frac{y}{x} + cosec (yx)\left(\frac{y}{x}\right) = 0 , y = 0 when x = 1
Let yx\frac{y}{x} = t ⇒ y = xt
dydx\frac{dy}{dx} = x dtdx\frac{dt}{dx} + t
By substituting dydx\frac{dy}{dx} in equation (i)
(xdtdx+t)\left(x\frac{dt}{dx}+t\right) - t + cosex t = 0
⇒ x dtdx\frac{dt}{dx} = - cosec t
⇒ ∫ dtcosect\frac{dt}{\text{cosect}} + ∫ dxx\frac{dx}{x} = 0
⇒ - cos t + log x = C ⇒ - cos (yx)\left(\frac{y}{x}\right) + log x = C
using y 0 when x 1
- 1 + 0 = C ⇒ C = - 1
So the solution is : cos (yx)\left(\frac{y}{x}\right) = log x + 1
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