餘弦換正弦

反例：

取 \(\displaystyle x_1 = 0, x_2 = \pi, x_3=x_4=\cdots=x_{2023}=\frac{\pi}{2}\)，則

\(\cos x_1+\cos x_2+\cdots+\cos x_{2023} = 0\)，

且 \(\sin x_1 +\sin x_2 + \cdots \sin x_{2023} \neq 2023\)