sinB+sinC=sinB+sin(π-A-B)=sinB+sin(A+B)=sinB+sinAcosB+cosAsinB
若cosA=4/5,则最大值=根号[(7/5)^2+(3/5)^2]=五分之根号五十八
若cosA=-4/5,则最大值=根号[(1/5)^2+(3/5)^2]=五分之根号十