# Thread: change of variables question in 1st order proof

1. ## change of variables question in 1st order proof

$\frac{dy}{dt}F(y(t))=g(t),\int_{to}^{t}\frac{dy}{d t}F(y(t))=\int_{to}^{t}g(t)=F(y(t))-F(yo), \int_{t}^{to}f(r)dr=\int_{to}^{t}g(s)ds,$
in differential equations when solving an initial value problem for c, you have this proof in Braun where he changes f(t) to f(r), g(t) to g(s)- I'm not sure what the reasoning is, maybe you can use any arbitrary variable? the time dependence of y on t also remains?

2. ## Re: change of variables question in 1st order proof

Hey mathlover10.

They are just dummy variables: my guess is that he is trying to avoid using the variables t, t0, y and y0 for the limits.

Remember that Integral 0 to x f(x)dx is a lot more complicated than Integral 0 to x f(s)ds. The second one is usually what is meant but sometimes you can get situations resembling the first (The Fresnel integrals come to mind).