在三角形ABC中∵cos(B+C)=cos(π-A)=-cosAcos(A+C)=cos(π-B)=-cosB又∵a*cos(B+C)=b*cos(A+C)∴a*(-cosA)=b*(-cosB)即a/b=cosB/cosA又∵根据正弦定理：a/sinA=b/sinB∴cosB/cosA=sinA/sinB即sinAcosA=sinBcosB∴sin...