sin(B+C)sin(B-C)=(sinBcosC+cosBsinC)(sinBcosC-cosBsinC)=sin^2Bcos^2C-cos^2Bsin^2C=sin^2B(1-sin^2C)-(1-sin^2B)sin^2C=sin^2B-sin^2C；cos(B+C)cos(B-C)=(cosBcosC-sinBsinC)(cosBcosC+sinBsinC)=cos^2Bcos^2C-...