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**格式**：
- 向量建议写成 $\boldsymbol{x}_{0}$
**内容**：
- 没有例题吗
=====知识点=====
====前言====
对于一元函数的极值问题相信大家都十分熟悉，但是对于多元函数的极值问题可能就会比较陌生。大家都学过淑芬怎么可能陌生呢
对于没有限制条件的多元函数来说，只需要对函数求导即可，但是若有了限制条件，即函数的值要在一定条件下才能取到，则需要用到拉格朗日乘子法。
====引理====
设函数 $f(\boldsymbol{x})$ ，${\boldsymbol{\varphi}}(\boldsymbol{x})=({\varphi}_1(\boldsymbol{x}),{\varphi}_2(\boldsymbol{x}),\cdots,{\varphi}_m(\boldsymbol{x}))$ 在区域 $D\subset \mathbb{R}^n (m 0$ 时显然取 $x+y+z=s$ 时会比 $x+y+z