正弦定理； a/sinA=b/sinB=c/sinC,
现在a/cos（A/2）=b/cos（B/2）=c/cos（C/2）,
所以 sinA / cos(A/2) = sinB / cos(B/2) = sinC / cos(C/2)
所以 sin(A/2) = sin(B/2) = sin(C/2)
（注意：A/2为什么 sinA / cos(A/2) = sinB / cos(B/2) = sinC / cos(C/2) 可以得出 sin(A/2) = sin(B/2) = sin(C/2)？倍角公式如：sin2A=2sinAcosA