即(x^2-8x+16)+(y^2+12y+36)=0
(x-4)^2+(y+6)^2=0
所以x-4=0,y+6=0
x=4,y=-6
所以原式=1/2x-1/(x-y)*[(x-y)/x-(x+y)(x-y)]
=1/2x-1/x+(x+y)
=-1/(2x)+x+y
=-1/8+4-6
=-17/8