Question 1005071
 I tried the formula using Cos(A+B)=CosA*CosB+SinA*SinB..plugged everything in and then added the two cos together to get Cos 14pi/18 then simplified to Cos 7pi/9 this is where am currently stuck at.

Edit: Oh I see my mistake, thanks... I added the 13pi and pi together instead of subtracting them.