cos(x+y)=cosxcosy−sinxsiny\cos (x + y) = \cos x \cos y - \sin x \sin ycos(x+y)=cosxcosy−sinxsiny
cos(60+45)=cos60cos45−sin60sin45\cos (60 + 45) = \cos 60 \cos 45 - \sin 60 \sin 45cos(60+45)=cos60cos45−sin60sin45
= 12×22−32×22\frac{1}{2} \times \frac{\sqrt{2}}{2} - \frac{\sqrt{3}}{2} \times \frac{\sqrt{2}}{2}21×22−23×22
= 2−64\frac{\sqrt{2} - \sqrt{6}}{4}42−6
