To prove the equation tan²θ - sin²θ = tan²θsin²θ, start by recalling the definition of tangent, tan(θ) = sin(θ)/cos(θ), which leads to tan²(θ) = sin²(θ)/cos²(θ). By substituting this into the equation, you can manipulate one side using trigonometric identities. The goal is to simplify the expression until both sides of the equation match. This involves algebraic manipulation and applying fundamental trigonometric relationships.