Which of these functions is strictly decreasing on $\left(0, \frac{\pi}{2}\right)$?

Explanation

1. 01

   $\frac{d}{dx}(\cos x) = -\sin x$, which is negative throughout $\left(0, \frac{\pi}{2}\right)$.

2. 02

   $\sin x$ and $\tan x$ rise there, and $\sin 2x$ rises up to $\frac{\pi}{4}$ and falls after it.