jika diketahui nilai cos(p+q) = 2/3 dan sin p sin q = -2/5, maka tentukan nilai cos(p+q)

Cos (p+q) = cos p cos q - sin p sin q
2/3 = cos p cos q - (-2/5)
2/3 = cos p cos q + 2/5
2/3 - 2/5 = cos p cos q
10/15 - 6/15 = cos p cos q
4/15 = cos p cos q