x^2-2(m-2)x+m^2=0
两个实数根
判别式delta = 4(m-2)^2 - 4m^2 >=0
得:m<=1
两个实数根设为a,b
a+b = 2(m-2)
ab = m^2
a^2 + b^2 = (a+b)^2 - 2ab = 2m^2-16m+16 = 56
m^2-8m-20 = 0
m = 10或-2
由于m<=1
则m = -2