1/4a^2+9b^2-a+12b+5=0
(1/4a^2-a+1)+(9b^2+12b+4)=0
(1/2a -1)^2+(3b+2)^2=0
所以a=2,b=-2/3
则(a-2)^2/a^2 -b^2
=-b^2
=-(-2/3)^2
=-4/9