1.(x+y)^2=36
x^2+y^2+2xy=36
x^2+y^2=36-2*2.75=30.5
(x-y)^2=x^2+y^2-2xy=30.5-2*2.75=25
x-y=5或者-5
2.m=-2
n=-5
3.(2+1)(2^2+1)(2^4+1).(2^2n+1)
=（2-1）(2+1)(2^2+1)(2^4+1).(2^2n+1)
=2^4n-1