Which of the following functions is decreasing on $\left(0,\tfrac{\pi}{2}\right)$?

1. For $\cos x$: $f'(x)=-\sin x<0$ on $\left(0,\tfrac{\pi}{2}\right)$, so it is decreasing. 2. For $\sin x$: $f'(x)=\cos x>0$, increasing. 3. For $\tan x$: $f'(x)=\sec^2 x>0$, increasing. 4. For $x^2$: $f'(x)=2x>0$, increasing. _Source: NCERT Class 12 Mathematics Ch 6 "Application of Derivatives", p.12_