a^2+b^2-8a+6b+25=0
a^2-8a+16+b^2+6b+9=0
(a-4)^2+(b+3)^2=0
因为(a-4)^2≥0
（b+3)^2≥0
所以a-4=0
b+3=0
a=4,b=-3
(2a^2-4ab)/(a^2-4ab+4b^2)
=2a(a-2b)/(a-2b)^2
=2a/(a-2b)
=(2*4)/(4+2*3)
=8/10
=4/5