a、b互为相反数,可得：a+b=0
m、n互为倒数,可得：mn=1
(a²-b²)÷(a²+b²)-√mn
=(a+b)(a-b)÷(a²+b²)-√mn
=0x(a-b)÷(a²+b²)-√1
=0-1
=-1