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链式法则

链式法则,台湾地区亦称连锁律(英语:Chain rule),用于求合成函数导数

正式表述

两函数 定义域 () 、值域 () 都包含于实数系 ,若可以定义合成函数 (也就是 ),且 可微分,且 可微分,则

也可以写成

例子

求函数 的导数。

求函数 的导数。

证明

严谨的证明需要以下连续函数的极限定理

都是实函数,若可以定义合成函数

则有


只要展开极限的δ-ε定义,并考虑 等于或不等于 的两种状况,这个极限定理就可以得证。

为了证明链式法则,定义一个函数 ,其定义域 , 而对应规则为

和一个函数 ,其定义域 , 而对应规则为

这样,考虑到 导数是以下函数(定义域为)的极限

因为可微则必连续(根据乘法的极限性质),所以 连续、 连续,故根据上面的极限定理有

而且针对一开始可微的前提有

再根据乘法的极限性质

即为所求。

多元复合函数求导法则

考虑函数z = f(x, y),其中x = g(t),y = h(t),g(t)和h(t)是可微函数,那么:

假设z = f(u, v)的每一个自变量都是二元函数,也就是说,u = h(x, y),v = g(x, y),且这些函数都是可微的。那么,z的偏导数为:

如果我们考虑

为一个向量函数,我们可以用向量的表示法把以上的公式写成f的梯度的偏导数的数量积

更一般地,对于从向量到向量的函数,求导法则为:

高阶导数

复合函数的最初几个高阶导数为:

参见

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链式法则
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