CBSE Class 12 Maths 2010 Solved Paper

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Question : 22
Total: 29
Find the general solution of the differential equation,
x log x
dy
dx
+ y =
2
x
log x
OR
Find the particular solution of the differential equation satisfying the given conditions:
dy
dx
= y tan x , given that y = 1 when x = 0.
Solution:  
x log x
dy
dx
+ y =
2
x
log x
Dividing all the terms of the equation by xlogx, we get
dy
dx
+
y
xlogx
=
2
x2

This equation is in the form of a linear differential equation
dy
dx
+ Py = Q , where { =
1
xlogx
and Q =
2
x2

Now, I.F. = ePdx = e
1
xlogx
d
x
= elog(logx) = log x
The general solution of the given differential equation is given by
y × I.F. = ∫ (Q × I.F.) dx + C
⇒ y log x = ∫ (
2
x2
l
o
g
x
)
dx
⇒ y log x = 2 ∫ (logx×
1
x2
)
dx
= 2 [log x × ∫
1
x2
dx - ∫ {
d
dx
(logx)
×
1
x2
d
x
}
dx]
= 2
[logx(
1
x
)
(
1
x
×(
1
x
)
)
d
x
]

= 2 [
logx
x
+
1
x2
d
x
]

= 2 [
logx
x
1
x
]
+ C
So the required general solution is y log x =
2
x
(1 + log x) + C
OR
dy
dx
= y tan x
dy
y
= tan x dx
On integration, we get
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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