tan a=sina / cosa =2 则 sina=2cos asina ^2 +cos a^2=1 即 4cos a*2+cos a ^2 =1 cos a^2= 1/5sin a ^2=4/5sin a ^2 +sina* cosa - 2cos a^2 = sina ^2 + 2 cos a * cos a - 2 cos a^ 2 =sina ^2 = 4/5