Hey Eraser147.

Try setting up a right angled triangle or using sums or differences formula to get the value.

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- December 11th 2012, 01:29 PM #1

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- December 12th 2012, 01:07 AM #2

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- December 13th 2012, 08:42 AM #3

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## Re: Cos 3pi/5

Consider the right-angled triangle with acute angles and and note that

To ease typing, let then

Use the standard identities,

and we have the cubic

is a factor, remove this and we are left with the quadratic

and solving this, (for the angle in the range),

Substituting this into (1) and simplifying leads to