diketahui alfa+beta =60° dan cos alfacos beta =1/4, maka cos (alfa -beta) = ....

Trigonometri

[tex] \alpha + \beta = 60 \\ \cos \alpha \cos \beta = \frac{1}{4} \\ \cos( \alpha + \beta ) = \cos \alpha cos \beta - \sin\alpha \sin\beta \\ \cos(60) = \frac{1}{4} - sin \alpha sinb \\ sin \: \alpha sin \: \beta = - \frac{1}{2} \\ \\ \cos( \alpha - \beta ) = \cos \alpha \cos \beta + sin \alpha \: sin \beta \\ = \frac{1}{4} - \frac{1}{2} = - \frac{1}{2} [/tex]