以为a、b、c成等比，所以b2=ac 原式=cosA/sinA+cosC/sinC=(sinCcosA+cosCsinA)/sinAsinC=sin(A+C)/sinAsinC=sinB/sinAsinC由正弦定理，a/sinA=b/sinB=c/sinC=2R 所以sinA=a/2R，sinB=b/2R sinC=c/2R 所以原式=b/2R*4R2/ac=2R/b=1/sinB 又因为 cosB=3/4 0
