parmenides51 wrote: Prove that for every acute angle $\alpha$, $\sin (\cos \alpha) < \cos(\sin \alpha)$. https://artofproblemsolving.com/community/c4h1447693p8283869 https://math.stackexchange.com/questions/2451169/how-can-i-prove-that-cos-sin-x-sin-cos-x-when-0-leq-x-leq-pi?noredirect=1