Question

Determine the exact value of cos18 (degrees). Hint:sin2x= cos3x where x=18 degrees.

Answer 1

cos(18)=0.951056516
If thats what you wanted

Answer 2

No, I am looking for the exact value.

Answer 3

using your hint you could also say
sin(4x) = cos(x) if x=18

Answer 4

using your identity
sin30 = cos3(18)
sin 30 = 0.5

Answer 5

cos3(18) = 0.5

Answer 6

no...cos(54) = .5877

Answer 7

yea - my mistake

Answer 8

http://en.wikipedia.org/wiki/Trigonometric_functions#Unit-circle_definitions

Answer 9

Thanks, but I still don't know how to come up with the answer...