a(x-x1)(x-x2)=ax^2-a(x1+x2)x+ax1x2又a(x-x1)(x-x2)=ax^2+bx+c故ax^2-a(x1+x2)x+ax1x2=x^2+bx+c经x^2的系数要相等(a=a),x的系数要相等(-a(x1+x2)=b),常数项系数要相等(ax1x2=c)整理即得：x1+x2=-b/ax1x2=c/a...