Question Details

Let y ( x ) be the solution of the differential equation
x 2 d y d x + x y = x 2 + y 2 , x > 1 e ,
satisfying y ( 1 ) = 0 .

Then the value of  2 y ( e ) 2 y ( e 2 ) is ______

Show Answer

Correct Answer :

0.75

Solution :

The correct answer is 0.75.

Step-by-step Derivation:

We are given the first-order differential equation:
x 2 d y d x + x y = x 2 + y 2
for x > 1 e , with the initial condition y ( 1 ) = 0 .

First, we divide the entire differential equation by x 2 to write it in homogeneous form:
d y d x + y x = 1 + ( y x ) 2

Let us use the substitution y = v x , which gives:
d y d x = v + x d v d x

Substituting these into the differential equation:
v + x d v d x + v = 1 + v 2
x d v d x = v 2 2 v + 1
x d v d x = ( v 1 ) 2

Separating the variables:
d v ( v 1 ) 2 = d x x

Integrating both sides:
d v ( v 1 ) 2 = d x x
1 v 1 = ln | x | + C

Substituting back v = y x :
1 y x 1 = ln | x | + C
x x y = ln | x | + C

Applying the initial condition y ( 1 ) = 0 :
1 1 0 = ln ( 1 ) + C C = 1

For x > 1 e , we obtain:
x x y = ln x + 1
x y x = 1 ln x + 1
1 y x = 1 ln x + 1
y x = 1 1 ln x + 1 = ln x ln x + 1
y ( x ) = x ln x ln x + 1

Note that the domain of the solution is restricted to x > 1 e . Thus, e 2 = 1 e 2 lies outside this domain. Evaluating the term in the denominator as y ( e 2 ) (which is the correct representation matching the domain constraint):

1. Find y ( e ) :
y ( e ) = e ln e
ln e + 1 = e 2

2. Find y ( e 2 ) :
y ( e 2 ) = e 2 ln ( e 2 ) ln ( e 2 ) + 1 = 2 e 2 2 + 1 = 2 e 2 3

3. Calculate the value of the expression:
2 [ y ( e ) ] 2 y ( e 2 ) = 2 ( e 2 ) 2 2 e 2 3 = 2 e 2 / 4 2 e 2 / 3 = 2 3 8 = 3 4 = 0.75

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