diketahui A+B = 2π / 3 dan cos A.Cos b 1/4 tentukan nilai cos (A-B) ???

[tex](a+ b) = \frac{2\pi}{3} \\ \cos(a + b) = \cos( \frac{2\pi}{3}) \\ \cos(a) \cos(b) - \sin(a) \sin(b) = - \frac{1}{2} \\ \frac{1}{4} - \sin(a) \sin(b) = - \frac{1}{2} \\ - \sin(a) \sin(b) = - \frac{3}{4} \\ \sin(a) \sin(b) = \frac{3}{4} \\ maka \\ \cos(a - b) = \cos(a) \cos(b) + \sin(a) \sin(b) \\ \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: = \frac{1}{4} + \frac{3}{4} = 1[/tex]