x²+y²-4x+6y+13=0(x-2)²+(y+3)²=0所以x-2=0 x=2y+3=0 y=-3x-y=2-(-3)=51/a-1/b=2(b-a)/ab=2a-b+2ab=0a-b=-2aba-b+ab=-ab2(a-b)-ab=-4ab-ab=-5ab(2a-ab-2b)/(a+ab-b)=(-5ab)/(-ab)=5