|
11 | 11 | "\n", |
12 | 12 | "> 例如,为了求和 $1 + 2 + \\cdots + 100$,高斯将首尾项配对(如 $1 + 100$, $2 + 99$ 等)从而得出总和。\n", |
13 | 13 | "\n", |
14 | | - "在这个解释中,我们将提供一种替代方法,揭示这种配对为何有效,重点在于平均值的对称性。\n", |
15 | | - "\n", |
16 | | - "## 数轴上平均值的对称性\n", |
17 | | - "\n", |
18 | | - "让我们重新思考在一个等差数列中平均值意味着什么,在这种数列中,每一项都增加一个相同的公差。\n", |
| 14 | + "在这个解释中,我们将提供一种替代方法,揭示这种配对为何有效,重点在于平均值的对称性。" |
| 15 | + ] |
| 16 | + }, |
| 17 | + { |
| 18 | + "cell_type": "markdown", |
| 19 | + "id": "d5d1fe39-640f-46cb-94ea-403eed950bfc", |
| 20 | + "metadata": {}, |
| 21 | + "source": [ |
| 22 | + "# 数轴上的对称平均值\n", |
19 | 23 | "\n", |
20 | | - "- 不仅仅是“总和除以项数”,\n", |
21 | | - "- 我们可以将平均值看作是一个中心点,周围的数值围绕它对称分布。\n", |
| 24 | + "让我们重新思考等差数列中平均值的含义,其中每一项都以相同的公差递增:\n", |
| 25 | + "- 不仅仅是\"总和除以项数\"\n", |
| 26 | + "- 我们可以将平均值视为数字对称平衡的中心点\n", |
22 | 27 | "\n", |
23 | 28 | "考虑任意一项:\n", |
24 | 29 | "\n", |
25 | 30 | "$$\n", |
26 | 31 | "\\large\\boxed{a}\n", |
27 | 32 | "$$\n", |
28 | 33 | "\n", |
29 | | - "如果我们向左和向右移动相同的距离 `d`,我们就可以得到两个点:\n", |
| 34 | + "如果我们从该项移动距离`d`,可以得到两个点:\n", |
30 | 35 | "\n", |
31 | 36 | "$$\n", |
32 | 37 | "\\large{\\boxed{a - d} \\longleftarrow a \\longrightarrow \\boxed{a + d}}\n", |
33 | 38 | "$$\n", |
34 | 39 | "\n", |
35 | | - "注意,我们构建的方式意味着 `a` 是这两个点的平均值,因为我们相对于 `a` 向左右各移动了相同距离。\n", |
| 40 | + "向相反方向移动相同距离时,`a`始终保持数值中心位置\n", |
| 41 | + "- 即**平均值**\n", |
36 | 42 | "\n", |
37 | 43 | "$$\n", |
38 | | - "\\colorbox{lightgray}{如果我们继续以相同距离向两边移动,`a` 将始终是平均值}\n", |
39 | | - "$$" |
40 | | - ] |
41 | | - }, |
42 | | - { |
43 | | - "cell_type": "markdown", |
44 | | - "id": "c0fae9eb-c5c4-41ac-b29a-7e99e504f90f", |
45 | | - "metadata": {}, |
46 | | - "source": [ |
47 | | - "# 递归构造\n", |
48 | | - "\n", |
49 | | - "让我们进一步说明这一点,把上面的情况想象成一个新的项 `b`:\n", |
50 | | - "\n", |
51 | | - "$$\n", |
52 | | - "\\large{b : \\boxed{\n", |
53 | | - " a - d \\longleftarrow a \\longrightarrow a + d\n", |
54 | | - " }\n", |
55 | | - "}\n", |
| 44 | + "\\large{\\textcolor{lightgray}{\\boxed{a - d}} \\textcolor{lightgray}{\\longleftarrow} a \\textcolor{lightgray}{\\longrightarrow} \\textcolor{lightgray}{\\boxed{a + d}}}\n", |
56 | 45 | "$$\n", |
57 | 46 | "\n", |
58 | | - "从 `b` 两侧各移动 `d` 距离:\n", |
| 47 | + "我们可以不断延续这个模式,保持`a`作为**平均值**\n", |
59 | 48 | "\n", |
60 | 49 | "$$\n", |
61 | | - "\\large{\n", |
62 | | - " \\boxed{b - d} \\longleftarrow b \\longrightarrow \\boxed{b + d}\n", |
63 | | - "}\n", |
| 50 | + "\\large{\\boxed{a - d} \\longleftarrow a \\longrightarrow \\boxed{a + d}}\n", |
64 | 51 | "$$\n", |
65 | 52 | "\n", |
66 | | - "这相当于:\n", |
67 | | - "\n", |
68 | 53 | "$$\n", |
69 | 54 | "\\large{ \\boxed{a - 2d} \\longleftarrow \\boxed{a - d} \\longleftarrow a \\longrightarrow \\boxed{a + d} \\longrightarrow \\boxed{a + 2d}}\n", |
70 | 55 | "$$\n", |
71 | 56 | "\n", |
72 | | - "请注意,我们可以不断这样做,`a` 总是位于中心,因为我们总是以**相等的距离**但**相反的方向**移动。\n", |
73 | | - "\n", |
74 | 57 | "$$\n", |
75 | | - "\\large{\n", |
76 | | - " \\dots \\longleftarrow \\boxed{a - d} \\longleftarrow \\boxed{a - d} \\longleftarrow a \\longrightarrow \\boxed{a + d} \\longrightarrow \\boxed{a + 2d} \\longrightarrow \\dots\n", |
77 | | - "}\n", |
| 58 | + "\\large{ \\textcolor{gray}{\\cdots \\boxed{a - 3d}} \\longleftarrow \\boxed{a - 2d} \\longleftarrow \\boxed{a - d} \\longleftarrow a \\longrightarrow \\boxed{a + d} \\longrightarrow \\boxed{a + 2d} \\longrightarrow \\textcolor{gray}{\\cdots \\boxed{a + 3d}}}\n", |
78 | 59 | "$$\n", |
79 | 60 | "\n", |
80 | | - "因此,即使没有与中心等距的配对存在,只要我们选择不同的 `d`,中心始终保持不变。\n", |
81 | | - "\n", |
82 | | - "$$\n", |
83 | | - "\\large{\n", |
84 | | - " \\dots \\longleftarrow \\boxed{a - 2d} \\textcolor{darkgray}{\\longleftarrow \\boxed{a - d}} \\longleftarrow a \\longrightarrow \\textcolor{darkgray}{\\boxed{a + d} \\longrightarrow} \\boxed{a + 2d} \\longrightarrow \\dots\n", |
85 | | - "}\n", |
86 | | - "$$\n", |
87 | | - "\n", |
88 | | - "归根结底,一切都取决于你选择的 `d` 是大还是小。\n", |
89 | | - "\n", |
90 | | - "因此:\n", |
| 61 | + "因此,\n", |
91 | 62 | "\n", |
92 | 63 | "$$\n", |
93 | | - "\\colorbox{lightgray}{中心(也是平均值)`a` 可以由任何两个相对于它对称的点推导而来}\n", |
| 64 | + "\\colorbox{lightgray}{如果我们持续向两个方向移动相同距离,`a`将始终保持为平均值}\n", |
94 | 65 | "$$" |
95 | 66 | ] |
96 | 67 | }, |
|
99 | 70 | "id": "c6ebcc2e-90fb-43ec-a528-6d825158dbb7", |
100 | 71 | "metadata": {}, |
101 | 72 | "source": [ |
102 | | - "# 平均值:寻找锚点\n", |
| 73 | + "# 寻找计算平均值的锚点\n", |
103 | 74 | "\n", |
104 | 75 | "任何两点关于中点`a`对称时,它们的**平均值**总是`a`:\n", |
105 | 76 | "\n", |
106 | 77 | "$$\n", |
107 | | - "\\dfrac{(a - 0) + (a + 0)}{2} = a\n", |
| 78 | + "\\dfrac{ \\textcolor{lightgray}{(} a \\textcolor{lightgray}{-} \\textcolor{lightgray}{0} \\textcolor{lightgray}{)} + \\textcolor{lightgray}{(} a \\textcolor{lightgray}{+} \\textcolor{lightgray}{0} \\textcolor{lightgray}{)} }{2} = a\n", |
108 | 79 | "$$\n", |
109 | 80 | "\n", |
110 | 81 | "$$\n", |
|
134 | 105 | "$$\n", |
135 | 106 | "\n", |
136 | 107 | "$$\n", |
| 108 | + "\\dfrac{(a - 2d) + (a + 2d)}{2} = a\n", |
| 109 | + "$$\n", |
| 110 | + "\n", |
| 111 | + "$$\n", |
137 | 112 | "\\dfrac{(a - d) + (a + d)}{2} = a\n", |
138 | 113 | "$$\n", |
139 | 114 | "\n", |
140 | 115 | "$$\n", |
141 | | - "\\dfrac{(a - 0) + (a + 0)}{2} = a\n", |
| 116 | + "\\dfrac{ \\textcolor{lightgray}{(} a \\textcolor{lightgray}{-} \\textcolor{lightgray}{0} \\textcolor{lightgray}{)} + \\textcolor{lightgray}{(} a \\textcolor{lightgray}{+} \\textcolor{lightgray}{0} \\textcolor{lightgray}{)} }{2} = a\n", |
142 | 117 | "$$\n", |
143 | 118 | "\n", |
144 | 119 | "由于**首项**和**末项**始终存在,\n", |
|
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