cos(x+y)=cosxcosy−sinxsiny\cos(x + y) = \cos x \cos y - \sin x \sin ycos(x+y)=cosxcosy−sinxsiny
cos(π2+π3)=cosπ2cosπ3−sinπ2sinπ3\cos\left(\frac{\pi}{2} + \frac{\pi}{3}\right) = \cos\frac{\pi}{2} \cos\frac{\pi}{3} - \sin\frac{\pi}{2} \sin\frac{\pi}{3}cos(2π+3π)=cos2πcos3π−sin2πsin3π
= (0×12)−(1×32)\left(0 \times \frac{1}{2}\right) - \left(1 \times \frac{\sqrt{3}}{2}\right)(0×21)−(1×23)
= 0−32=−320 - \frac{\sqrt{3}}{2} = -\frac{\sqrt{3}}{2}0−23=−23
