分享知识、传播快乐、增长见闻、留下美好，大家好，这里是Leanringyard学院。今天以及后面的文章，小编将持续为大家带来Mathematica入门教程系列文章。今天的主题就是：Mathematica学习3。
Share knowledge, spread happiness, increase knowledge and leave beauty. Hello, everyone, this is leanringyard college. In today's and later articles, Xiaobian will continue to bring you a series of Mathematica introductory tutorials. Today's topic is: getting started with Mathematica tutorial 2.
特殊矩阵输入/Special matrix input上一次，我们学习了一般矩阵的输入，本期我们将来学习几个比较特殊的矩阵输入：
1.海塞矩阵
海塞矩阵是由一个多元函数的二阶偏导构成的方阵用于描述函数的局部曲率，可以用来判断多元函数的极值。
首先要将多元函数求二阶导，然后由二阶偏导数构成方阵，同样使用矩阵格式（MatrixForm）让其输出为矩阵。
Last time, we learned the input of general matrix. In this issue, we will learn several special matrix inputs:
1. Hesse matrix
Hessian matrix is a square matrix composed of the second-order partial derivative of a multivariate function. It is used to describe the local curvature of the function and can be used to judge the extreme value of the multivariate function.
First, the second derivative of the multivariate function is obtained, and then the square matrix is formed by the second partial derivative. Similarly, the matrix format is used to output it as a matrix.
2.雅可比矩阵
雅可比矩阵是由多个函数的一阶偏导数按照一定方式排列的方阵，它表示出一个可微方程与给出的点的最优线性逼近。与海塞矩阵的区别在于它是求一阶导之后再用矩阵格式输出。
2. Jacobian matrix
Jacobian matrix is a square matrix arranged in a certain way by the first-order partial derivatives of multiple functions. It represents the optimal linear approximation between a differentiable equation and the given points. The difference from Hessian matrix is that it is output in matrix format after obtaining the first derivative.
上面学习了矩阵，那么对于括号的使用就必不可少，接下来就一起看看怎么样合理而正确的使用。
首先，括号的种类有方括号[]、花括号{}、双方括号[[]]、圆括号（）。
每一种括号的使用场景都有不同，也有相同。
After learning the matrix above, it is essential to use parentheses. Next, let's see how to use them reasonably and correctly.
First, the types of brackets include square brackets [], curly brackets {}, square brackets [[]], and parentheses ().
The usage scenarios of each bracket are different and the same.
1.方括号用于软件内部函数以及用户自定义函数的自变量和参数。从下图可以看出[]是y用于求导函数D的。
2.花括号可以用来表示一个表，以及参数范围、界限、集合等。在上面我们就是用了花括号用来表示矩阵。如果不习惯这种表示方式，则可以使用矩阵格式将其转化为我们常见的方阵形式。
3.双方括号可以用来表示某一元素。比如在矩阵中提取出某行某列的元素。
Square brackets are used for arguments and parameters of software internal functions and user-defined functions.
Curly braces can be used to represent a table, as well as parameter ranges, bounds, sets, etc. In the above, we use curly braces to represent the matrix.
Brackets can be used to represent an element. For example, extract the elements of a row and a column from the matrix.
4.圆括号大家比较熟悉，大部分都是用来表示运算顺序。
4. Parentheses are familiar to us. Most of them are used to indicate the operation order.
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