  In this paper we obtain a new curious identity involving trigonometric
functions. Namely, for any positive odd integer $n$ we show that
$$\sum_{k=1}^n(-1)^k(\cot kx)\sin k(n-k)x=\frac{1-n}2.$$ Consequently, for any
positive odd integer $n$ and positive integer $m$ we have
$$\sum_{k=1}^n(-1)^kk^{2m}B_{2m+1}\left(\frac{n-k}2\right)=0,$$ where $B_j(x)$
denotes the Bernoulli polynomial of degree $j$.

Combinatorics;Number Theory