Question

if sin 1 + sin 2 + sin 3 = 3, then cos1 cos2 + cos 3?​

Answer 1

Sin1 +Sin2 + Sin3 =3

The maximum value of sin(x) is 1. The value of 3 can only be obtained if all the sin angle takes the value of 1.

So, Sin1 = Sin2 = Sin3 = 1

We can conclude that

1 ~ 2 ~ 3 ~ 90°

So, cos1 + Cos2 + cos3 = cos90° + Cos90° + cos90°= 0 + 0 + 0 = 0