Using the cosine of a sum formula: $\cos(\alpha\pm\beta)=\cos(\alpha)\cos(\beta)\mp\sin(\alpha)\sin(\beta)$, where angle $\alpha$ equals $\frac{\pi }{2}$, and angle $\beta$ equals $x$
The sine of $\frac{\pi }{2}$ equals $1$
Multiply $-1$ times $1$
The cosine of $\frac{\pi }{2}$ equals $0$
Any expression multiplied by $0$ is equal to $0$
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