已知sina·cosa=2/5,则tana=________.

已知sina·cosa=2/5,则tana=________.
已知sina·cosa=2/5,则tana=________.

已知sina·cosa=2/5,则tana=________.
sina·cosa=2/5,sinacosa/(sin^2a+cos^2a)=2/5
左端分子分母 同除以cos^2a:tana/(1+tan^2a)=2/5,2tan^2a-5tana+2=0,tana=2或0.5