｛x+y=4-m
xy+m(x+y)=5
xy+m(4-m)=5
x(4-m-x)+m(4-m)=5
-x^2+(4-m)x-(m^2-4m)-5=0
x^2+(m-4)x+(m^2-4m+5)=0
判别式=(m-4)^2-4*1*(m^2-4m+5)
=m^2-8m+16-4m^2+16m-20
=-3m^2+8m-4
方程组｛x+y=4-m xy+m(x+y)=5 有实数解
则有判别式>=0
-3m^2+8m-4>=0
3m^2-8m+4