1.1-x+4分之x^2
=(1-x/2)^2
2.（x-y)^2-4(x-y-1)
=(x-y)^2-4(x-y)+4
=(x-y-2)^2
m^2+(n-mn)^2+2m(n-mn)
=(m+n-mn)^2
=(3-2/3)^2
=(7/3)^2
=49/9