tanα=2sanα/cosα=2sinα=2cosαsin²α+cos²α=14cos²α+cos²α=1cos²α=1/5α是第三象限角则：cosα=-√5/53-sinα/1+cosα=(3+2√5/5)/(1-√5/5)=(15+2√5)/(5-√5)=(15+2√5)(5+√5)/(25-5)=(75+10+25√5)/20=(17-5√5)/4
17+5（根号5）/4
