I have been trying to solve a differential equation as a practice question for my test, but I am jus

letumsnemesislh

letumsnemesislh

Answered question

2022-06-29

I have been trying to solve a differential equation as a practice question for my test, but I am just unable to get the correct answer. Please have a look at the D.E:
d y / d x = 1 / ( 3 x + sin ( 3 y ) )
My working is as follows:
d x / d y = 3 x + sin ( 3 y )
d x 3 x = sin ( 3 y ) d y
Integrating both sides:
x ( 3 / 2 ) x 2 = ( 1 / 3 ) cos ( 3 y ) + c
But the correct answer is:
x = c e 3 y 1 / 6 ( cos ( 3 y ) + sin ( 3 y ) ), which is quite different from what I have got. Could someone please help me solve this?

Answer & Explanation

Alec Blake

Alec Blake

Beginner2022-06-30Added 11 answers

HINT.....You need to use the integrating factor method.
Starting with
d x d y 3 x = sin 3 y
The integrating factor is
I = e 3 d y = e 3 y
Therefore
x e 3 y = e 3 y sin 3 y d y
Frederick Kramer

Frederick Kramer

Beginner2022-07-01Added 7 answers

The step you tried to do would have to read d x = 3 x d y + sin ( 3 y ) d y, which is a bad sign: you have a function of x multiplied with d y which means the variables have not been separated. You also tried to integrate a 3 x that had no d x, which does not make proper sense.
Instead of trying to think about it like separation of variables, notice that your new equation (after changing variables to x ( y ) instead of y ( x )) is linear, so you should solve it by the method of integrating factors. That is, you should have d d y ( e 3 y x ) = e 3 y sin ( 3 y ) and now you just have an integral and some algebra to do.

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