tanα=2 ,secα=±√5 ,cosα=±√5/5 ,sinα=±2√5/5
cos（2α+π）=-cos2α=1-2cos^2α=3/5
sin（π-α）cos（-α）=sinαcosα=±2/5可是∵tanα=2∴cos（2α+π）=-cos2α=-1-tan2α1+tan2α=351-tan2α1+tan2α=35什么意思？1+tan2a分之1-tan2a=5分子3(1-tan2a)/(1+tan2a)=3/55(1-tan2a)=3(1+tan2a)5-5tan2a=3+3tan2a8tan2a=2tan2a=1/4