因为（a+2)^2+(3b-1)^2=0，又(a+2)^2>=0, (3b-1)^2>=0，所以a=-2, b=1/3.3a^2b-(2ab^2-6(ab-1/2a^2b)+4ab)-2ab=-2ab^2=-2*(-2)*(1/3)^2=4/9
