Let \( f_n=\displaystyle\int\limits_{0}^{\pi/2}\left(\sum_{k=1}^{n}\sin^{k-1}x\right)\left(\sum_{k=1}^{n}(2k-1)\sin^{k-1}x\right)\cos x \, dx,\; n\in \mathbb{N} \). Then \( f_{21}-f_{20} \) is equal to ______.