Answer:
a) 1/2, b) = - sqrt(3) / 2
Step-by-step explanation:
a) Let A = 120 degree, B = 180 degree
Using cosine sum is cos(120+180) = cos(120)*cos(180) - sin(120)* sin(180)
which will give you 1/2
b) Let A = 360, B = 60 degree
Using sine difference: sin(360-60) = sin(360) cos(60) - cos(360) sin (60)
gives you -sqrt(3)/2