已知 cos(a+b)=1/3 ,cos(a-b)=1/5 ,则tan a 乘tan b 等于已知 cos(a+b)=1/3 ,cos(a-b)=1/5 则tan a 乘tan b 等于 A1/4 B-1/4 C4 D -4
网友回答
cos(a+b) = 1/3
cosa cos b - sina sinb = 1/3 ---(1)
cos(a-b)=1/5
cosa cosb + sina sinb = 1/5 ---(2)
(1) + (2)
2cosa cosb = 8/15 --(3)
(2) - (1)
2 sina sinb = -2/15 ---(4)
(4)/(3)
sinasinb/cosacosb = -1/4
tana.tanb = -1/4
ans :B======以下答案可供参考======
供参考答案1:
B供参考答案2:
B