马上,码字中、、、ok(1)sinC=sin(180-A-B)=sin(A+B)=sinAcosB+cosAsinB.
sinC-sinBcosA= sinAcosB+cosAsinB -sinBcosA=sinAcosB=0,
因 0
（2）(cosA/2)^2 = (2√5/5)^2 =4/5
cosA=2(cosA/2)^2-1 = 2*4/5 -1= 3/5, sinA=√[1-(cosA)^2] =√[1-(3/5)^2] =4/5
sinC=cosA=3/5
a/(b+c) = sinA/(sinB+sinC)=4/5 / (1+3/5) = 1/2