sinAcosC+sinCcosA=4sinCcosA
sin(A+C)=4sinCcosA
sinB=4sinCcosA
根据正弦定理和余弦定理
b/2R=4(c/2R)*(b^2+c^2-a^2)/2bc
(1/2)b^2=b^2+c^2-a^2
又a^2-c^2=2b
(1/2)b^2=b^2-2b
b(b-4)=0
b=4