# Equations with dependent variable missing

1. Feb 9, 2016

### Joseph1739

Information from the book:
Use
1. v = y'
2. v' = y''
3. dy/dt = v
to solve the differential equation.

Question: 2t2y'' + (y')3 = 2ty'
I'm stuck at finding the integrating factor (which my book tells me is v-3 in the solutions.)

Using the information above:
2t2v' + (v^3) - 2tv = 0
M(t,v) = v3 - 2tv
N(t,v) = 2t2
Mv = 3v2 - 2t
Nt = 4t
Since these two are not equal, I have to find an integrating factor, but using both:
(1) dμ/dt = [(Mv-Nt)/(N)] * μ
(2) dμ/dv = [(Nt-Mv)/(M)] * μ
both result in a unsolvable equation because I end up with both t's and v's in the equation.

2. Feb 10, 2016

### haruspex

I'm not familiar with this M, N, notation, so I cannot follow your method.
I would start by considering it dimensionally. If v is dimensionally some power n of t, what value of n makes the equation dimensionally consistent? That should suggest a change of variable that simplifies it a bit.
See how you go from there.

3. Feb 14, 2016

### Ssnow

The equation $2t^2v'-2tv=-v^3$ can be written as $v'-\frac{v}{t}=-\frac{v^3}{2t^2}$ , that is a Bernoulli equation of type:

$y'(x) + f(x)y=g(x)y^{n}$

here $n=3, x=t, y=v, f=-\frac{1}{t}, g=-\frac{1}{2t^2}$. To solve it see the link

https://en.wikipedia.org/wiki/Bernoulli_differential_equation

4. Feb 15, 2016

### HallsofIvy

Staff Emeritus
The equation $2t^2v'= 2tv- v^3$ can e written $2t^2 dv= (2tv- v^3)dt$ and then $2t^2dv+ (v^3- 2tv)dt$. Many textbooks write the "generic" first order differential equation $M(x,y)dy+ N(x,y) dx= 0$ or, with variables v and t rather than y and x, $M(v,t)dv+ N(v,t)dt$.
Further, such an equation is "exact" (and so particularly easy to solve) if and only if $\frac{\partial M}{\partial x}= \frac{\partial N}{\partial y}$ or, in terms of v and t, $\frac{\partial M}{\partial t}= \frac{\partial N}{\partial v}$. That is where Joseph1739 got "$M= 2t^2$" and "$N= 2tv- v^3$. Of course, as he pointed out, $\frac{\partial M}{\partial t}= 4t\ne 2t- 3v^2= \frac{\partial N}{\partial v}$ so this is not an exact equation.