用待定系数法求二次函数解析式时,根据条件特点通常选用一般式y=ax~2+bx+c或顶点式y=a(x-h)~2+k或两根式y=a(x-x_1)(x-x_2).当条件中有抛物线上的两个对称点(x_1,m)、(x_2,m)时,可设解析式为y=a(x-x_1)(x-x_2)+m.这就是二次函数的对称点式,易知,当m =0时,对称点式就变成为两根式. When using the undetermined coefficient method to solve the analytical expression of the quadratic function, the general formula y=ax~2+bx+c or the vertex y=a(xh)~2+k or the two formulas y=a(x- X_1) (x-x_2). When there are two symmetrical points (x_1, m), (x_2, m) on the parabola in the condition, the analytical expression can be set to y=a(x-x_1)(x-x_2) +m. This is the symmetry point of the quadratic function. It is easy to know that when m = 0, the symmetry point becomes two types.