Hey Eraser147.

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

Results 1 to 3 of 3

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

- Joined
- Oct 2012
- From
- Nowhere
- Posts
- 82

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

- Joined
- Sep 2012
- From
- Australia
- Posts
- 5,967
- Thanks
- 1558

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

- Joined
- Jun 2009
- Posts
- 671
- Thanks
- 136

## 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