α,β都是锐角cosa=√(1-sin²a)=3/5sinb=√(1-cos²b)=12/13∴sin(a-b)=sinacosb-cosasinb=4/5×5/13-3/5×12/13=-16/65sin2a=2sinacosa=24/25cos2a=cos²a-sin²a=-7/25cos(2a-b)=cos2acosb+sin2a...