x^2-(m-2)x+2m-7
x^2-2*(1/2)(m-2)x+[(1/2)(m-2)]^2-[(1/2)(m-2)]^2+2m-7
x^2-2*(1/2)(m-2)x+[(1/2)(m-2)]^2-(1/4)(m^2-4m+4)+2m-7
x^2-2*(1/2)(m-2)x+[(1/2)(m-2)]^2-(1/4)m^2+m-1+2m-7
x^2-2*(1/2)(m-2)x+[(1/2)(m-2)]^2-(1/4)m^2+3m-8
因是完全平方,所以-(1/4)m^2+3m-8=0
m^2-12m+32=0
(m-4)(m-8)=0
m1=4,m2=8