sinα/cosα=1/2
cosα=2sinα
代入sin²α+cos²α=1
所以sin²α=1/5
cos²α=4/5
sin2α
=2sinαcosα
=2sinα(2sinα)
=4sin²α
=2/5
cos2α
=cos²α-sin²α
=3/5
tan2α
=sin2α/cos2α
=2/3