由正弦定理:
b/2R=sinB,
c/2R=sinC
所以(b+c)/2c
=[(2RsinB)+(2RsinC)]/[2(2RsinC)]
=(sinB+sinC)/2sinC
所以:
cos^2(A/2)=(sinB+sinC)/2sinC
(cosA+1)/2=(sinB+sinC)/2sinC
(cosA+1)sinC=sinB+sinC
cosAsinC=sinB
=sin(π-A-C)
=sin(A+C)
=sinAcosC+cosAsinC
所以sinAcosC=0
因为A是三角形内角，所以sinA>0
故cosC=0
C=90°
所以三角形是直角三角形
