Because this is a zero distribution problem, as long as you recite a zero in the m n interval, it is easy to understand, that is, look at the symmetry axis, the endpoint value, the open direction of the quadratic function, and the discriminant δ. As long as a is greater than zero and f(m) or f(n) is less than 0, there must be two unequal real roots, and δ must be greater than 0, so δ does not need to be considered. When a is greater than 0, it is the axis of symmetry. So you don't have to consider the axis of symmetry. The terminal value fm multiplied by fn is less than 0, and a is greater than 0. There must be a 0 o'clock in the interval of m n, it's as simple as that. Other things need to consider the position of the axis of symmetry.