1. The problem statement, all variables and given/known data Recast the integration variable of the following integral to da [tex]\tau[/tex]=[tex]\int[/tex]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.