Math Help - Help - Sum of Cosines

1. Help - Sum of Cosines

I need help with the sum of cosines. Specifically this problem:

Cos(2pi/3 + pi/4)

Detail is highly appreciated. Thank you in advance.

2. Originally Posted by Neversh
I need help with the sum of cosines. Specifically this problem:

Cos(2pi/3 + pi/4)

Detail is highly appreciated. Thank you in advance.
cos (a + b ) = cos(a)cos(b) - sin(a)sin(b)

a= 2pi/3

b = pi/4

...and you definitely know the sines and cosines of these two angles.

Good luck!

3. Had to finish dinner. So, the awnser would be: $(-sqrt(2) - sqrt(6))/4$ ?

4. Originally Posted by Neversh
Had to finish dinner. So, the awnser would be: $(-sqrt(2) - sqrt(6))/4$ ?
correct.