Question

How do you prove cos(2x) = ... using the unit circle?

Answer 1

the trig identity ?

cos(2x) = cos^2(x) - sin^2(x)

Answer 2

Right, how is it derived using the unit circle?

Answer 3

hmm i know it comes from angle sum formula

cos(a+b) = cos(a)cos(b) - sin(a)sin(b)

...don't remember how they derive it though.. some fancy geometry i believe

Answer 4

right, it is the fancy geometry I am looking for

Answer 5

here you go

http://en.wikipedia.org/wiki/Proofs_of_trigonometric_identities#Angle_sum_identities