已知cos(a+b)=1/3 cos(a-b)=1/5 求tana*tanb

问题描述:

已知cos(a+b)=1/3 cos(a-b)=1/5 求tana*tanb
答案是1/4我算的是负1/4……为什么

cos(a+b)=1/3.(1)
cos(a-b)=1/5 .(2)
coscosb-sinasinb = 1/3 .(3)
coscosb+sinasinb = 1/5 .(4)
{(4)-(3)}÷2 得:sinasinb = -1/15 .(5)
{(4)+(3)}÷2 得:cosacosb = 4/15 .(6)
(5)÷(6)得:tana*tanb = -1/4
答案错了!