a^2+b^2+4a-2b+5
=a^2+4a+4+b^2-2b+1
=(a+2)^2+(b-1)^2
即(a+2)^2+(b-1)^2=0
非负数之和零,那么,两个非负数都为零
a+2=0,b-1=0
a=-2,b=1
(a+b)/(a-b)=-1/-3=1/3
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