Question

Please help! 15degrees (using 45degrees and 30degrees)
cos15degrees=

a. cos 45° cos 30° + sin 45° sin 30°
b. sin 45° cos 30° + cos 45° sin 30°
c. cos 45° cos 30° - sin 45° sin 30°
d. sin 45° cos 30° - cos 45° sin 30°

Answer 1

15 = 45 - 30

Then use the identity

cos(A-B) = cos(A)cos(B) + sin(A)sin(B)

Answer 2

in this case, A = 45 and B = 30

Answer 3

Thanks :)