The secant line is only an approximation to the behavior of the function at the point a because it does not account for what happens between a and a + h.
割线仅仅是函数在 a {\displaystyle a} 点行为的一个近似，因为它不能解释函数在 a {\displaystyle a} 到 a + h {\displaystyle a+h} 之间的情况。