# Recasting integration variable

1. Jun 2, 2009

1. The problem statement, all variables and given/known data

Recast the integration variable of the following integral to da

$$\tau$$=$$\int$$k dr

k is a positive constant

2. Relevant equations

dr= c*dt/a

3. The attempt at a solution

I have no idea, I'm sure it's something really simple, but I really just can't think.

btw, this is part of a bigger problem, that I know how to solve, but I've forgotten how to do this.

Thanks.

Last edited: Jun 2, 2009
2. Jun 3, 2009

### tiny-tim

Welcome to PF!

Do you mean dt rather than da?

If not, can you tell us the bigger problem, so we can see what it all means?

3. Jun 3, 2009

### HallsofIvy

Staff Emeritus
If, as tiny tim suggested, you meant "recast to dt", then just replace the "dr" in the integral with "cdt/a" which you say is equal to it.

4. Jun 3, 2009