cos(x+y)=cosxcosy-sinxsiny=3/5 (1)
cos(x-y)=cosxcosy+sinxsiny=4/5 (2)
(1)+(2) 2cosxcosy=7/5 (3)
(2)-(1) 2sinxsiny= 1/5 (4)
（4）/ (3)
tanx*tany=1/7
COS(X加Y)＝3/5,cos(x减y)=4/5.则tanx乘tany
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