x^2–2（m+1）x+m^2+5=0
[x - (m+1)]^2 -(m+1)^2 + m^2+5 = 0
完全平方，则 -(m+1)^2 + m^2+5 = 0
所以 -(m^2+2m+1) + m^2+5 =0
4-2m=0
m=2
