# Another Mathematica Question about Solve Function

1. Mar 17, 2009

Okay, so I have this algebraic expression in terms of P,G,J,K...etc

I want to solve it for the expression P/G (where I have the "?" below in the code). But Mathematica won't let me do this in the solve function. It says that P/G is not a valid variable name.

How can I get around this and solve for P/G ?

Code (Text):
Solve[((P*L^3)/(3*H*
J)) - ((5*L^3)/(48*H*J))*((40 L^3 M P)/(384 H J + 5 L^3 M)) -
G == 0, ?]

Last edited: Mar 17, 2009
2. Mar 17, 2009

Come on now... I know somebody's got this one

3. Mar 17, 2009

### Winzer

$$\frac{P L^3}{3 H J}-\left(\frac{5 L^3}{48 H J}right)\left(\frac{40 L^3 M P}{384 H J + 5 L^3 M}\right)-G=0$$
Can't you just factor out the P?

4. Mar 17, 2009

Well, yes, of course. I can; but, I want Mathematica to do that for me

5. Mar 18, 2009

### NeoDevin

Add another equation, X = P/G, and solve the system of 2 equations for X? Worth a try.

6. Mar 18, 2009

### Dick

I used to do a lot of Mathematica. Don't anymore. Two g*dd**m expensive. But what does the '?' do in your statement? Can't you specify a variable to solve for in place of that? Like Solve[G+H/P==0,P]. That's what the documentation seems to indicate.

7. Mar 18, 2009

The ? is what I want to solve for, which is not a single variable...it is an expression = P/G

8. Mar 18, 2009

### Dick

Ohhhhhh. I see. Finally. How about something along the lines of NeoDevin's suggestion. Substitute G*X for P in the expression and then solve for X.

9. Mar 21, 2009