1, let z=bi, then (z+2) ÷ (1-I) is a pure imaginary number, and b=2, that is, z = 2i.

2.[2x+ 1/x] 6, and the expansion constant term is c (3,6) [2x]? [ 1/x]？ = 160;

3、2x？ -3x ≤ 0, that is, 0≤x≤3/2, and the maximum value is19/4 when x=3/2;

4、(-3,- 1]