Question Details

Let y : (-∞, ∞) → (0, ∞) be the solution of the differential equation dydx=e5xy3+y3ex+exy4 satisfying y(0)=12. Then the value of y(loge 2) is

Options

A

5+352

B

7+532

C

7+532

D

5+352

Show Answer

Correct Answer :

Option B

7+532

Solution :

The correct option is 7+532.

Step 1: Simplify and separate variables in the differential equation.
The given first-order ordinary differential equation is:

dydx=e5xy3+y3ex+exy4

Factor out common terms in the numerator and the denominator:

dydx=y3(e5x+1)ex(1+y4)

Rearrange terms to separate the variables y and x:

1+y4y3dy=e5x+1exdx

Simplify each side by dividing term-by-term:

(y-3+y)dy=(e4x+e-x)dx

Step 2: Integrate both sides.
Integrating both sides with respect to their corresponding variables:

(y-3+y)dy=(e4x+e-x)dx

Evaluating the integrals yields:

-12y2+y22=e4x4-e-x+C

Multiply the entire equation by 4 to clear the fractions:

2y2-2y2=e4x-4e-x+K

where K=4C is an arbitrary constant.

Step 3: Apply the initial condition.
We are given that y(0)=12. Substitute x=0 and y=12 into the equation:

Since y2=12:

2(12)-21/2=e0-4e0+K

1-4=1-4+K

-3=-3+KK=0

Thus, the relationship simplifies to:

2y2-2y2=e4x-4e-x

Step 4: Find the value of y(loge2).
Substitute x=loge2 into the equation:

e4x=e4loge2=eloge(24)=16

e-x=e-loge2=eloge(1/2)=12

Substituting these exponential values into the equation:

2y2-2y2=16-4(12)=16-2=14

Divide by 2:

y2-1y2=7

Let t=y2 (where t>0 since y>0):

t-1t=7t2-7t-1=0

Solve this quadratic equation using the quadratic formula:

t=-(-7)±(-7)2-4(1)(-1)2(1)=7±49+42=7±532

Since t=y2>0 and 53>7, we discard the negative root to preserve positivity:

y2=7+532

Taking the positive square root (since y>0):

y(loge2)=7+532

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