CBSE Class 12 Math 2013 Solved Paper

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Question : 26
Total: 29
Show that the differential equation 2yexy dx + (y - 2x exy) dy is homogeneous. Find the particular solution of this differential equation, given that x = 0 when y = 1.= 0
Solution:  
2yexy dx + (y - 2x exy) dy = 0
dx
dy
=
2xe
x
y
y
2ye
x
y
... (1)
Let f (x , y) =
2xe
x
y
y
2ye
x
y

Then, (λx , λy) =
λ(2xe
x
y
y
)
λ(2ye
x
y
)
= λ0 [F (x,y)]
Thus, F(x, y) is a homogeneous function of degree zero. Therefore, the given differential equation is a homogeneous differential equation.
Let x = vy
Differentiating w.r.t. y, we get
dx
dy
= v + y
dv
dy

Substituting the value of x and
dx
dy
in equation (1), we get
v + y
dv
dy
=
2vyevy
2yev
=
2vev1
2ev

or y
dv
dy
=
2vev1
2ev
- v
or y
dv
dy
= -
1
2ev

or 2ev dv = -
dy
y

or ∫ 2ev . dv = - ∫
dy
y

or 2ev = - log |y| + C
Substituting the value of v, we get
2e
π
y
+ log |y| = C ... (2)
Substituting x = 0 and y = 1 in equation (2), we get
2e0 + log |1| = C ⇒ C = 2
Substituting the value of C in equation (2), we get
2e
x
y
+ log |y| = 2, which is the particular solution of the given differential equation.
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