(2a-3b)(3a-2b)+(4a-3b)^2+(a-b)(-a-b)
=6a^2-13ab+6b^2+16a^2-24ab+9b^2-a^2+b^2
=21a^2-37ab+16b^2(将a=1/2 b=-1代入)
=21*(1/2)^2-37*1/2*(-1)+16*(-1)^2
=21/4+37/2+16
=39又3/4