I dont even know how to describe that... :S finding an exact value of cos36??

This one says: Noting that t=pi/5 satisfies 3t=pi-2t , fins the exact value of cos36.....This identities may be uself: cos^2t+sin^2t=1 , sin2t=2sintcost sin3t=3sint-4sin^3t Any clues???

Re: I dont even know how to describe that... :S finding an exact value of cos36??

Finding the exact value of is the same as finding the same value of , because .

You are given formulae for sin, so we're going to use them:

But we can also do this:

Since is the same no matter how we write it:

, that means we can simplify the relation by .

Let be , so .

So .

PS: I think this thread should be here.