已知sinθ-cosθ=1/2,則sin²;θ-cos²;θ=()

已知sinθ-cosθ=1/2,則sin²;θ-cos²;θ=()

sina-cosa=1/2
(sina-cosa)²;=1/4
1-2sinacosa=1/4
sin2a=3/4
cos2a=±根號下7/4
sin²;a-cos²;a=-cos2a=±根號下7/4