How do I find cos2(5pi/6), cos^2(5pi/6), and cos(-5pi/6)?

Answer 1

cos[2(5π/6] = cos(5π/3) = cos(π/3) = 1/2

Using the half angle formula for cosine, cos²(5π/6) = [cos((5π/3)/2)]² = [√(1 + cos(5π/3)/2)]² = [√(3/4)]² = 3/4

Since cosine is an even function, cos(-5π/6) = cos(5π/6) = √3/2