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Question: Answered & Verified by Expert
 If x3dy+xy·dx=x2dy+2ydx; y2=e and x>1, then y4 is equal to :
MathematicsDifferential EquationsJEE Main
Options:
  • A e2
  • B 12+e
  • C 32e
  • D 32+e
Solution:
2541 Upvotes Verified Answer
The correct answer is: 32e

Given x3dy+xy·dx=x2dy+2ydx

x3-x2dy=2-xydx

dyy=2-xx3-x2dx

Integrating both sides with respect to x, we get

dyy=2-xx2x-1dx+k ............i

Let 2-xx2x-1=Ax+Bx2+Cx-1

2-x=Axx-1+Bx-1+Cx2

Putting x=02=-BB=-2

Putting x=12-1=CC=1

Putting x=22-2=A21+B1+C222A+2=0A=-1

From equation i, we get

dyy=-1x+-2x2+1x-1dx+k

lny=-lnx+2x+lnx-1+k ..........ii

Given y2=e i.e. at x=2, y=e

lne=-ln2+22+ln2-1+k

1=-ln2+1+0+k

k=ln2

Putting in equation ii, we get

lny=-lnx+2x+lnx-1+ln2

Now putting x=4, we get

lny=-ln4+24+ln4-1+ln2

lny=-2ln2+12+ln3+ln2

lny=-ln2+12+ln3

lny=12+ln32

ln2y3=12

2y3=e12

y=32e

y4=32e

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