Determine the interval(s) in which \(f(x)=5x^{3/2}-3x^{5/2}\), \(x>0\) is increasing.
Find the sub-interval of \((0, \infty)\) in which \(f(x) = x \log x\) is increasing.
Find the sub intervals in which \(f(x) = \cot^{-1}(\sin x + \cos x),\ x \in (0, \pi)\) is increasing and decreasing.