Find exact value using compound or double angle formula
tan(-15*)
I'm sure you know the values for 30°, and now you just need to apply the half-angle formula
tan(θ/2) = (1-cosθ)/sinθ = sinθ/(1+cosθ)
tan(-15*)
tan(θ/2) = (1-cosθ)/sinθ = sinθ/(1+cosθ)