Question

Use a double angle or half angle identity to find the exact value of each expression

Answer 1

θ is given to be in the fourth quadrant (270° < θ < 360°) for which sin(θ) < 0 and cos(θ) > 0. This means

cos²(θ) + sin²(θ) = 1   ==>   sin(θ) = -√[1 - cos²(θ)] = -3/5

Now recall the double angle identity for sine:

sin(2θ) = 2 sin(θ) cos(θ)

==>   sin(2θ) = 2 (-3/5) (4/5) = -24/25