1.1. Vectors and Linear Combinations向量和线性组合
emmm,想写细一点,发现下面的概括很准确了,没必要
REVIEW OF THE KEY IDEAS
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A vector v in two-dimensional space has two components v1 and v2.(二维空间中的向量v,有2个分量v1和v2)
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$v + w = ( v1 + w1, v2 + w2) $and (cv = ( cv1, cv2)) are found a component at a time.(向量的加法和数乘,每次计算出一个分量)
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A linear combination of three vectors u and v and w is cu+ dv + ew.(向量的线性组合)
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Take all linear combinations of u, or u and v, or u, v, w. In three dimensions,
those combinations typically fill a line, then a plane, then the whole space (R^3) .(在三维的情况下,u、u和v、u,v和w的全部线性组合,分别能够填满一条线、一个平面和一个三维空间)
1.2 Lengths and Dot Products向量长度和点积
emmm,想写细一点,发现下面的概括很准确了,没必要
REVIEW OF THE KEY IDEAS
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The dot product (v • w) multiplies each component (v_i) by (w_i) and adds all (v_iw_i). (2个向量的点积 = 对应分量乘积之和)
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The length (||v||) is the square root of (v · v). Then (u = v / ||v||) is a unit vector : length 1.(向量长度是向量自身点积的算术平方根,单元向量的长度是1)
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The dot product is (v · w) = 0 when vectors (v) and (w) are perpendicular.(2个向量互相垂直时,点积为0)
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The cosine of θ ( the angle between any nonzero v and w) never exceeds 1(任意2个非0向量夹角的余弦值都不超过1):
Cosine (cosθ = frac{v · w}{||v||||w||}), Schwarz inequality (|v·w|≤||v||||w||)
1.3 Matrices矩阵
矩阵只是一种表示方式,一种形式,重要的是:矩阵中元素的解释方式、矩阵间操作的含义,矩阵和向量间操作的含义。
某种意义上来说,可以理解为:解线性方程组的时候,嫌麻烦,想少写点字。。。因此简化成了矩阵形式。。。
一些和矩阵密切相关的概念(简单知道就行,公开课也没在第一章讲这些,而是在后面的章节细说):
- Linear Equations线性方程组
- The Inverse Matrix逆矩阵
- Cyclic Differences循环差
- Independence and Dependence线性无关与线性相关
REVIEW OF THE KEY IDEAS
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Matrix times vector: A(x) = combination of the columns of A(矩阵 × 向量 = 矩阵中列向量的线性组合)
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The solution to A(x) = b is (x) = (A^{-1}b), when A is an invertible matrix.(如果A是可逆矩阵,那么Ax=b方程的解是:x=(A^{-1}b))
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The cyclic matrix C has no inverse. Its three columns lie in the same plane.(循环矩阵C没有逆矩阵,它的3个列向量在同一个平面上)
Those dependent columns add to the zero vector. Cx = 0 has many solutions.(线性相关的列向量和为0向量时,有非0解)
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This section is looking ahead to key ideas, not fully explained yet.(1.3这个小节有点超前了。os:确实,很多概念在公开课上都是后面会详细讲的,这部分目前只需要简单知道就行,讲太细会丧失学习兴趣的)