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CBSE Class 12 Maths 2010 Solved Paper

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Question : 22 of 29
Marks: +1, -0
Find the general solution of the differential equation,
x log x dydx\frac{dy}{dx} + y = 2x\frac{2}{x} log x
OR
Find the particular solution of the differential equation satisfying the given conditions:
dydx\frac{dy}{dx} = y tan x , given that y = 1 when x = 0.
Solution:  
x log x dydx\frac{dy}{dx} + y = 2x\frac{2}{x} log x
Dividing all the terms of the equation by xlogx, we get
dydx+yxlogx\frac{dy}{dx} + \frac{y}{x \log x} = 2x2\frac{2}{x^2}
This equation is in the form of a linear differential equation
dydx\frac{dy}{dx} + Py = Q , where { = 1xlogx\frac{1}{x \log x} and Q = 2x2\frac{2}{x^2}
Now, I.F. = ePdxe^{\int P \, dx} = e1xlogxdxe^{\int \frac{1}{x \log x} \, dx} = elog(logx)e^{\log(\log x)} = log x
The general solution of the given differential equation is given by
y × I.F. = ∫ (Q × I.F.) dx + C
⇒ y log x = ∫ (2x2logx)\left( \frac{2}{x^2} \log x \right) dx
⇒ y log x = 2 ∫ (logx×1x2)\left( \log x \times \frac{1}{x^2} \right) dx
= 2 [log x × ∫ 1x2\frac{1}{x^2} dx - ∫ {ddx(logx)×1x2dx}\left\{ \frac{d}{dx} (\log x) \times \int \frac{1}{x^2} \, dx \right\} dx]
= 2
[logx(1x)(1x×(1x))dx]\left[ \log x \left( -\frac{1}{x} \right) - \int \left( \frac{1}{x} \times \left( -\frac{1}{x} \right) \right) dx \right]
= 2 [logxx+1x2dx]\left[ -\frac{\log x}{x} + \int \frac{1}{x^2} \, dx \right]
= 2 [logxx1x]\left[ -\frac{\log x}{x} - \frac{1}{x} \right] + C
So the required general solution is y log x = 2x-\frac{2}{x} (1 + log x) + C
OR
dydx\frac{dy}{dx} = y tan x
dyy\frac{dy}{y} = tan x dx
On integration, we get
dyy\frac{dy}{y} = ∫ tan x dx
⇒ log y = log (sec x) + log C ... (1)
⇒ log y = log (C sec x)
⇒ y = C sec x
Now,it is given that y = 1 when x = 0
⇒ 1 = C × sec 0
⇒ 1 = C × 1
∴ C = 1
Substituting C = 1 in equation (1), we get
y = sec x as the required particular solution.
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