General Formula using Miscellaneous Substitution

asteg123
Messages
12
Reaction score
0
I'm really stomped with this problem... i can't seem to get the answer...

anyway... here's the problem..

(2(x^3) - (y^3))y'=3(x^2)y

and i need to get the general solution...

SO, here's what i did...

I Let
u=x^3 and
du=3x^2dx

so what happens is

2udy-(y^3)dy= ydu

and that's where i got stuck...

i tried using
d(y/u)=(udy-ydu)/u^2

but i can't seem to get rid of the 2 in 2udy and it would be much of a problem if i did that...

could anyone help me??
 
Physics news on Phys.org
Rewrite this diff.eq as:
y'=\frac{3\frac{y}{x}}{2-(\frac{y}{x})^{3}}\equiv{F}(\frac{y}{x}

I'm sure you have solved such problems previously.
 
There are two things I don't understand about this problem. First, when finding the nth root of a number, there should in theory be n solutions. However, the formula produces n+1 roots. Here is how. The first root is simply ##\left(r\right)^{\left(\frac{1}{n}\right)}##. Then you multiply this first root by n additional expressions given by the formula, as you go through k=0,1,...n-1. So you end up with n+1 roots, which cannot be correct. Let me illustrate what I mean. For this...
Back
Top