Question

Use the angle sum identity to find the exact value of each

1. Cos 105 degrees

2. Sin 195 degrees

Answer 1

hints:
105 degrees = 60+45 degrees
195 degrees = 135+60 degrees.
And use sin(x)=sin(180-x).
For values of sin(45), cos(45), sin(60), cos(60), use the unit circle.
The sum formula is
cos(a+b)=cos(a)cos(b)-sin(a)sin(b)
sin(a+b)=sin(a)cos(b)+cos(a)sin(b).