Question

how do you put this cos^2(pi/11)-sin^2(pi/11) in double-angle formula?

Answer 1

well its cos(pi/11 + pi/11)

Answer 2

because
cos(2A) = cos(A +A)
= cos(A) cos(A) - Sin^(A)sin^(A)
= cos^2(A) - sin^(A)

Answer 3

Thank you!
What is an angle between 0 and 2pi that is coterminal with -5pi/6?