a/c=sinA/sinC,b/c=sinB/sinC,
acosB/c-bcosA/c=4/5,
sinAcosB/sinC-sinBcosA/sinC=4/5,
(sinacosB-sinBcosA)/(sinacosB-sinBcosA)=4/5,
(tanA-tanB)/(tanA+tanB)=4/5,同除以tanB
tanA/tanB=9.
a/c=sinA/sinC,b/c=sinB/sinC,
acosB/c-bcosA/c=4/5,
sinAcosB/sinC-sinBcosA/sinC=4/5,
(sinacosB-sinBcosA)/(sinacosB-sinBcosA)=4/5,
(tanA-tanB)/(tanA+tanB)=4/5,同除以tanB
tanA/tanB=9.