根据正弦定理:(a-c)/(b-c)=sinB/(sinA+sinC)=b/(a+c)a²=b²+c²-bc根据余弦定理:a²=b²+c²-2bccosAcosA=0.5A=π/3 先降次在和差化积:f(x)=√3/2*cos2x递增区间[(k-1)π/2, kπ/2]