cariin dong cos a pakai jalannya ya.....

2²+4²-2(2)(4).cos a = 6²+8²-2(6)(8).-cos a
4+16-16cos a = 36+64+96cos a
20 - 16cos a = 100 + 96cos a
-16cos a - 96cos a = 100 - 20
- 112 cos a = 80
cos a = 80/-112 = -10/14