Posted by Hannah on Wednesday, March 17, 2010 at 11:25pm.
but it gave you a hint, ...
cos(A+B) = cosAcosB - sinAsinB
does the right side not follow the pattern of your question?
so ...
cos pi/9 cos pi/3 - sin pi/9 sin pi/3
= cos(pi/9 + pi/3)
= cos(4pi/9)
You are right, 4pi/9 radians, or 80 degrees, is not one of the standard angles on the unit circle, nor is it obtainable using combinations of our standard angles.
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