cos(a+b)=cosacosb-sinasinb=1/4
cos(a-b)=cosacosb+sinasinb=2/3
所以cosacosb=(1/4+2/3)/2=11/24
sinasinb=(2/3-1/4)/2=5/24
tana*tanb=(sinasinb)/(cosacosb)=(5/24)/(11/24)=5/11