cos 100.cos 20 - sin 100.sin 20

[tex]rumu s \: cos( \alpha + \beta ) = \cos\alpha . \cos \beta - \sin\alpha \sin\beta [/tex]jika cos 100.cos 20 -sin 100.sin 20
maka
cos ( 100+20 )
cos 120
-1/2
[tex] - \frac{1}{2} \sqrt{3} [/tex]