# Confused on simple differential equation

1. Aug 30, 2016

### BillhB

1. The problem statement, all variables and given/known data
Find the values of m so that $y = x^m$ is a solution of $x^2\frac{d^2y}{dx^2} - 3x\frac{dy}{dx} -12y = 0$

2. Relevant equations
$y = x^m$
$y'=mx^{m-1}$
$y''=(m^2-m)x^{m-2}$

3. The attempt at a solution
So after plugging and chugging we get

$$(m+2)(m-6)x^m = 0$$

Henceforth m = -2 or m = 6.

All well and good but why don't we look at the case when x = 0. Then the domain of m would be $(-\infty, 0) \cup (0, \infty)$ right?

The question doesn't state we're looking for m's for any possible x, so this seems correct to me. Is it just assumed that is the case? Or am I misunderstanding something?

Last edited: Aug 30, 2016
2. Aug 30, 2016

### tommyxu3

The problem required to find the value of $m$ such that $y=x^m$ is the solution, so I think your $m$ has to make all the $x$ and $y$ fit the equation.
Besides, $x$ varies and what is given is a function. I think $x=0$ just meets a special case. (my opinion)

3. Aug 30, 2016

### BillhB

I agree but I put when X = 0 m equaled the above ... when $X \neq 0$ m = -2 or m =6 and got points deducted with a red mark through the when X = 0 part???? Wondering if something about above is wrong or if I need to go to professor and seek clarification for deduction.

4. Aug 30, 2016

### tommyxu3

Ummm... Then I think his viewpoints are that the value $m\in \mathbb{R}\setminus\{0\}$ cannot make all the $x$ fit the equation, in which case we cannot say $y=x^m$ is a solution to the ODE.

5. Aug 30, 2016

### Staff: Mentor

The goal of the exercise is to find a function y = xm that satisfies the DE. As your work shows, two possible functions are y = x-2 and y = x6.

When you substitute the equations $y = x^m$, $y' = mx^{m - 1}$ and $y'' = m(m - 1)x^{m - 2}$ into the original diff. equation, you get $(m + 2)(m - 6)x^m = 0$. This equation has to be true for all real x, or at least all x values in the domains of the solutions, not just for a specific x value of 0. For this reason, you don't consider the possibility that xm = 0.

6. Aug 30, 2016

### BillhB

Got you. Thanks.