向量m=(sinB,1-cosB),向量n=(2,0),
m•n=2sinB,
|m|=√(sin²B+(1-cosB) ²)=√(2-2 cosB)= √[2(1- cosB)]= √[2•2sin²(B/2)]=2 sin(B/2).
|n|=2
所以Cosα=m•n/(|m||n|)=2sinB/[4 sin(B/2)]= 4 sin(B/2)cos(B/2) /[4 sin(B/2)]= cos(B/2).
由已知：Cosα=1/2,
∴cos(B/2) =1/2,B/2 =π/3.B=2π/3.
由正弦定理得a/sinA=b/sinB=c/sinC=2R=2.
所以(a +c )/(sinA +sinC)=2
a +c=2(sinA +sinC)
∵B=2π/3.A +C=π/3.
∴a +c=2(sinA +sin(π/3-A))=2(sinA +√3/2cosA-1/2sinA)
=2(1/2sinA +√3/2cosA)=2sin (A+π/3)
因?yàn)?