Cos 345 derajat + cos 105 derajat = ?

Cos 345° + cos 105° =
2 cos 1/2 (A+B) cos 1/2 (A-B) =
2 cos 1/2 (345+105) cos 1/2 (345-105) =
2 cos 1/2 (450) cos 1/2 (240) =
2 cos 225 cos 120
2×(-√2/2) × (-1/2)
-√2 × -1/2
√2/2

semoga bisa membantu!! terima kasih