(1/tanA)+(1/tanC)=cosA/sinA+ cosC/sinC=sin(A+C)/(sinAsinC)=sinB/(sinAsinC)
由正弦定理得sinB/(sinAsinC)=b/(ac)
因为abc成等比,故b/(ac)=1/b=1/sinB
所以(1/tanA)+(1/tanC)=(1/sinB)
(1/tanA)+(1/tanC)=cosA/sinA+ cosC/sinC=sin(A+C)/(sinAsinC)=sinB/(sinAsinC)
由正弦定理得sinB/(sinAsinC)=b/(ac)
因为abc成等比,故b/(ac)=1/b=1/sinB
所以(1/tanA)+(1/tanC)=(1/sinB)