# Simplify, evaluate if possible using the unit circle: #cos(pi/9)cos(pi/18)-sin(pi/9)sin(pi/18)# ?

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Simplify, evaluate if possible using the unit circle:

#cos(pi/9)cos(pi/18)-sin(pi/9)sin(pi/18)#

I can put this in all sorts of formats using half and double angle formulas, but I don't know what is the "most simple" or how to evaluate it using the unit circle. Help?

Thanks!!

Simplify, evaluate if possible using the unit circle:

I can put this in all sorts of formats using half and double angle formulas, but I don't know what is the "most simple" or how to evaluate it using the unit circle. Help?

Thanks!!

##### 1 Answer

May 6, 2018

See below

#### Explanation:

Most simple technique would be to recognize that this is the cosine sum identity:

So: