x^2-(a+2)x+2a<0,
[x-(a+2)/2]^2+2a-a^2/4-a-1<0
[x-(a+2)/2]^2-(a^2/4-a+1)<0
[x-(a+2)/2]^2<[(a-2)/2]^2
若a>=2,则-(a-2)/2若a<2,则(a-2)/2所以a<=3