Originally Posted by

**sqleung** Hello. Sorry to be a nuisance to you guys but you've all been very helpful. This is going to be my last question.

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If:

sin(3pi / 10) = (1 + squareroot 5) / 4

Find the exact value of **cos(pi / 5)**

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This is the second part of a question. The first part mentioned something about *sin(2pi / t) = cos t* so I guess I have to somehow incorporate this identity. You don't have to solve it (although you could if you want to) but it would be great if you could provide me a starting point so I could work my way through. I was thinking that perhaps a unit circle diagram may help.

Thankyou very much. Of course, all help is appreciated. (happy)