CBSE Class 12 Math 2012 Solved Paper

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Question : 22
Total: 29
Find the particular solution of the following differential equation:
(x + 1)
dy
dx
= 2ey - 1 ; y = 0 when x = 0
Solution:  
(x + 1)
dy
dx
= 2ey - 1
dy
2ey1
=
dx
x+1

ey
2ey
=
dx
x+1

Integrating both sides, we get:
eydy
2ey
= log |x + 1| + log C ... (1)
Let 2 - ey = t
d
dy
(2ey)
=
{dt}/{dy}$$
⇒ - ey =
dt
dy

ey dy = - dt
Substituting this value in equation 1 , we get:
∫ -
dt
t
= log |x + 1| + log C
⇒ - log |t| = log |C (x + 1)|
⇒ - log |2 - ey| = log |C (x + 1)|
1
2ey
= C (x + 1)
⇒ 2 - ey =
1
C(x+1)
... (2)
Now, at x = 0 and y = 0, equation 2 becomes:
⇒ 2 - 1 =
1
C

⇒ C = 1
Substituting C = 1 in equation 2 , we get:
2 - ey =
1
x+1

ey = 2 -
1
x+1

ey =
2x+21
x+1

ey =
2x+1
x+1

⇒ y = log |
2x+1
x+1
|
, (x ≠ - 1)
This is the required particular solution of the given differential equation.
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