|A+2|+(B-1/4)^2=0
因为：|A+2|≥0,(B-1/4)^2≥0
所以：|A+2|=0,(B-1/4)^2=0
由：|A+2|=0,得：A+2=0或-A-2=0,
即：A=-2
有：(B-1/4)^2=0,得：B-1/4=0
即：B=1/4
[(AB)^2-2AB]-3AB
=(AB)^2-5AB
=(-2×1/4)^2-5×(-2)×1/4
=1/4+5/2
=11/4