SOLUTION: Find the general solution of this linear differential equation: (x^4 +2y)dx-xdy=0

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Question 1143624: Find the general solution of this linear differential equation:
(x^4 +2y)dx-xdy=0

Answer by Edwin McCravy(20056) About Me  (Show Source):
You can put this solution on YOUR website!
%28x%5E4+%2B2y%29dx-xdy=0
We cannot separate the variables, so we hope that it is linear.
It is linear if we can get it in the form

dy%2BP%28x%29y%2Adx=Q%28x%29%2Adx

then we can solve it by multiplying by the
integrating factor matrix%282%2C1%2C%22%22%2Ce%5Eint%28P%28x%29%2Adx%29%29

So let's try to get it in that form:

%28x%5E4+%2B2y%29dx-xdy=0

x%5E4%2Adx%2B2y%2Adx-xdy=0

-xdy%2B2y%2Adx=-x%5E4%2Adx

Divide through by -x

-xdy%2F%28-x%29%2B2y%2Adx%2F%28-x%29=%28-x%5E4%2Adx%29%2F%28-x%29

dy%2B%28-2%2Fx%29y%2Adx=x%5E3%2Adx

So it is a linear differential equation with P%28x%29=%28-2%2Fx%29=-2x%5E%28-1%29 and Q%28x%29=x%5E3

Linear differential equations are usually easier if we can
avoid denominators by using negative exponents:

dy%2B%28-2x%5E%28-1%29%29y%2Adx=x%5E3%2Adx

We calculate the integrating factor

matrix%282%2C1%2C%22%22%2Ce%5Eint%28P%28x%29%5E%22%22%2Adx%29%29matrix%282%2C1%2C%22%22%2C%22%22=%22%22%29matrix%282%2C1%2C%22%22%2Ce%5Eint%28%28-2x%5E%28-1%29%29%2Adx%29%29matrix%282%2C1%2C%22%22%2C%22%22=%22%22%29matrix%282%2C1%2C%22%22%2Ce%5E%28-2%2Aint%28%281%2Fx%29%2Adx%29%29%29matrix%282%2C1%2C%22%22%2C%22%22=%22%22%29matrix%282%2C1%2C%22%22%2Ce%5E%28-2%2Aln%28x%29%29%29matrix%282%2C1%2C%22%22%2C%22%22=%22%22%29matrix%282%2C1%2C%22%22%2Ce%5E%28ln%28x%5E%28-2%29%29%29%29matrix%282%2C1%2C%22%22%2C%22%22=%22%22%29matrix%282%2C1%2C%22%22%2Cx%5E%28-2%29%29%29

We multiply through by the integrating factor x%5E%28-2%29

x%5E%28-2%29%2Ady%2Bx%5E%28-2%29%28-2x%5E%28-1%29%29y%2Adx=x%5E%28-2%29%2A%28x%5E3%2Adx%29

x%5E%28-2%29%2Ady%2B%28-2x%5E%28-3%29%29y%2Adx=x%2Adx%29

We integrate both sides:

int%28%28x%5E%28-2%29%2Ady%2B%28-2x%5E%28-3%29%29%5E%22%22y%2Adx%29%29%22%22=%22%22int%28x%5E%22%22%5E%22%22%2Adx%29%29

The right side is easy to integrate.  The left side requires a little
more thinking since we cannot integrate the terms separately.  On the left 
side, we have the integral of the differential of a product 
d(u*v) = u*dv+v*du, where u=x%5E%28-2%29 and v=y, so the left side
integrates to the product u%2Av or x%5E%28-2%29%2Ay and so the general
solution is

x%5E%28-2%29y=x%5E2%2F2%5E%22%22%2BC

Multiply both sides by x²

x%5E2%2Ax%5E%28-2%29y=x%5E2%2Aexpr%28x%5E2%2F2%5E%22%22%29%2Bx%5E2%2AC

x%5E0%2Ay=expr%28x%5E4%2F2%5E%22%22%29%2BCx%5E2

y=x%5E4%2F2%5E%22%22%2BCx%5E2

Edwin