tanα+1/tanα=2.5,则cos2α=

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tanα+1/tanα=2.5,则cos2α=

tana+1/tana=2.5=5/2
2tan^2a-5tana+2=0
(2tana-1)(tana-2)=0
tana=2或1/2
cos2a=(1-tan^2a)/(1+tan^2a)
当tana=2时,cos2a=-3/5
当tana=1/2时,cos2a=3/5
则cos2a=-3/5或3/5


tana+1/tana=2.5=5/2
2tan^2a-5tana+2=0
(2tana-1)(tana-2)=0
tana=2或1/2
cos2a=(1-tan^2a)/(1+tan^2a)
当tana=2时，cos2a=-3/5
当tana=1/2时,cos2a=3/5
则cos2a=-3/5或3/5