Let y = y ( x ) be the solution of the differential equation 3 y 2 - 5 x 2 y d x + 2 x x 2 - y 2 d y = 0…

Let y=y(x) be the solution of the differential equation 3y2-5x2ydx+2xx2-y2dy=0 such that y(1)=1. Then (y(2))3-12y(2) is equal to :
  1. 64
  2. 322
  3. 32
  4. 162

Solution

Given,

3y2-5x2ydx+2xx2-y2dy=0

dydx=y5x2-3y22xx2-y2

This is homogeneous differential equation

Let y=txdydx=t+xdtdx

t+xdtdx=(tx)5x2-3t2x22xx2-t2x2=t5-3t221-t2

xdtdx=5t-3t32-2t2-t=3t-t32-2t2

21-t23t-t3dt=1xdx

Taking integration on both sides we get,

21-t23t-t3dt=1xdx......1

Now let 3t-t3=z31-t2dt=dz

1-t2dt=13dz

Substitute in equation 1 we get,

23dzz=1xdx

23ln(z)=lnx+lnc

z23=cxz=c32x32

3t-t3=c32x32

Now put t=yx

3yx-y3x3=c32x32

Given y(1)=1 when x=1,y=1

 c32=2

So, equation becomes 3yx-y3x3=2x32

3x2y-y3=2x92......2

Put x=2 in equation 2,

12y(2)-y(2)3=2(2)92=322

y(2)3-12y(2)=322

Asked in: JEE Main 2023 (31 Jan Shift 2)

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