tana=2,为正数,说明sina与cosa符号相同sina^2为正数,sin2a=2sinacosa也为正数因为tana=2所以sina/cosa=2sina=2cosasina^2=4cosa^2sina^2=4(1-sina^2)sina^2=4/5cosa^2=1/5前面已经说过sin2a为正数sin2a=2sinacosa=2*...