(x-y)七次方÷【(x-y)三次方÷(y-x)】三次方
=(x-y)七次方÷【-(x-y)²】三次方
=(x-y)七次方÷【-(x-y)六次方】
=-(x-y)
=y-x
m+1+2n-1=5
n+2+2m=3
∴m=-1
n=3
∴m+n=2