[1CB] Prerequisites:[1C8]βΊβ».
Let \(Iβ β\) be an open interval. Let \(f:Iββ\) be a differentiable function such that \(f'(t)β 0\) for every \(tβ I\): show then that \(f'(t)\) has always the same sign.
[1CC]βΊβ»