Trigonometry- Find the exact value of cos 7pi/12 using half angle identities. I'm not too good at doing these when they want the value of a fraction with pi :p What're the steps to follow through with this?

Question

anonymous · Answer

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anonymous · Answer

cos(7pi/12)=cos((3pi+4pi)/12)=cos(pi/3+pi/4)=cos(pi/3)*cos(pi/4)-sin(pi/3)*sin(pi/4)=(sqrt(2)-sqrt(6))/4

anonymous · Answer

How'd you know to use 3p/4? :o

anonymous · Answer

Sorry,  I meant 3pi/4

anonymous · Answer

3pi/4??!!

zzr0ck3r · Answer

because those are angles we "should" have memorized. In other words you look for something you know:)

anonymous · Answer

Blah! Lol Sorry, I'm looking at it wrong. I ment (3pi+4pi)/12

anonymous · Answer

Oooh, okay (:

anonymous · Answer

That equation is a little cluttered. What do you do next once you've picked or gotten your "known" angles?

anonymous · Answer

sorry my english is not well!!!!