use identities to find the value of each expression

Question

If cos( pi/2 - Θ ) = -0.21, find sin( -Θ)

Answer 1

Since cos(π/2-θ) = sin(θ), we have sin(θ)=-0.21. Since sin is an odd function, sin(-θ) = -sin(θ). This gives us sin(-θ) = -sin(θ) = -(-0.21) = +0.21