1. The problem statement, all variables and given/known data J(a, b, c;y)=∫aydx/√((x-a)(x-b)(x-c)), let a<b<c 2. Relevant equations f(θ, k)=∫0θdx/√(1-k2sin2(x)), k≤1 3. The attempt at a solution This is an example from my study material, and I don't understand the first step they do. Let x=a+(b-a)t, dx=(b-a)dt Wait...what? Why? How did they come to that decision. I hope there's a logical reason, otherwise how can I apply this to my other problems? I keep running into "make an educated guess." Is this another guess?