已知tana=-1/2,则2sinacosa/sin^2a-cos^2a的值是

2sinacosa/(sin²a－cos²a)＝sin2a/(?cos2a)＝?tan2a＝?2tana/(1－tan²a) ＝?2×(?1/2)/[1－(?1/2)²]＝1/(3/4)＝4/3