Q55 / 90 · JEE Main 2022 (25 Jul Shift 2) · Mathematics

Let y=yx be the solution of the differential equation dydx=4y3+2yx23xy2+x3,y1=1. If for some n∈N,y2∈[n-1,n), then n is equal to _______.

Correct answer: 3

Solution

Given,

dydx=yx4y2+2x23y2+x2

Let y=vx

⇒dydx=v+xdvdx

So, dydx=yx4y2+2x23y2+x2

⇒v+xdvdx=v4v2+23v2+1

⇒xdvdx=v4v2+2-3v2-13v2+1

⇒∫3v2+1dvv3+v=∫dxx

⇒lnv3+v=lnx+c

⇒lnyx3+yx=lnx+c

Given, y1=1

⇒c=ln2

So the equation becomes lnyx3+yx=lnx+ln2

Now for y2

So putting x=2 in lnyx3+yx=lnx+ln2

 We get, lny38+y2=2ln2⇒y38+y2=4

⇒y2=2

So, y2∈[2,3)

So on comparing with y2∈[n-1,n)

⇒n=3

Hence, the correct answer is 3.

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