cos2a/sin(a-π/4)=-根号2/2 ∴ sin(π/2-2a)/sin(a-π/4)=-根号2/2 ∴ sin(2a-π/2)/sin(a-π/4)=根号2/2 即 2cos(a-π/4)=√2/2∴ 2[cosacos(π/4)+sinasin(π/4)]=√2/2∴ √2( cosa+sina)=√2/2(1)∴ sina+cosa=...