This is the time derivative to calculate the speed which a galaxy moves away from another galaxy. I don't understand how they get from da/dt (xi − x1) to (∙a)/a a(t). (xi − x1). Could anyone explain this? vi(t) = d/dt (ri(t) − r1(t))
= d/dt a(t)(xi − x1)
= da/dt (xi − x1)...