x^2分之1+y^2分之1=n,
所以两边平方得：x+y+2x^(1/2)y^(1/2)=n^2
所以x+y=n^2-2m^(1/2)
所以(x-y)^2=(x+y)^2-4xy=n^4+4n^2m^(1/2)