cos2a/sin(a-π/4)=-√2/2（cos²a-sin²a)/(sinacosπ/4-cosasinπ/4)=-√2/2∴[(cosa+sina)(cosa-sina)]/[√2/2(sina-cosa)]=-√2/2∴-√2 (cosa+sina)=-√2/2∴sina+cosa=1/2