证明：a(x-x1)(x-x2)
=a(x-(-b+sqrt(b*b - 4ac))/a/2)(x-(-b-sqrt(b*b - 4ac))/a/2)
=a(x*x+bx/a+c/a)
=ax*x + bx + c
其中sqrt（x）= √（x）,即取根号