tana = 2sin²a = 4/5cos²a = 1/5sina cosa = 2/5sin²a + sina cosa - 2cos²a = 4/5 + 2/5 - 2/5 = 4/5
已知tana=2，所以sina=2/√5，cosa=1/√5，则sin^2a+sina*cosa-2cos^2a=4/5+2/5-2/5=4/5.
