sin(α+β)=sinacosb+cosasinb=1/2
sin(α—β)=sinacosb-cosasinb=1/3
两式相加得：sinacosb=5/12
两式相减得：cosasinb=1/12
所以,
1/tanα*tanβ=sinbcosa/cosbsina=(1/12) / (5/12)=1/5
log以五为底tanβ/tanα的对数=log5(1/5)=-1