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<article class="md-content__inner md-typeset">
<h1 id="_1">函数逼近<a class="headerlink" href="#_1" title="Permanent link">&para;</a></h1>
<p><em>函数逼近用足够接近原函数的简单函数来替代复杂函数。本文涵盖线性化、泰勒级数、多项式逼近、傅里叶级数以及通用逼近定理——这些是神经网络能够学习任意映射的理论基础。</em></p>
<ul>
<li>
<p>我们遇到的许多函数都过于复杂,无法直接处理。例如,在纸上计算 <span class="arithmatex">\(e^{0.1}\)</span>、预测卫星轨迹等,都涉及没有简单封闭形式答案的函数。</p>
</li>
<li>
<p><strong>函数逼近</strong>用更简单的函数来替代复杂函数,使其在关心区域内"足够接近"原函数。</p>
</li>
<li>
<p>最自然的逼近是多项式。多项式只是 <span class="arithmatex">\(x\)</span> 的幂次与系数的和,易于求值、微分和积分。</p>
</li>
<li>
<p>但为什么多项式作为逼近器如此有效?看看 <span class="arithmatex">\(x\)</span> 的每个幂次贡献了什么。</p>
<ul>
<li>常数项 <span class="arithmatex">\(a_0\)</span> 设定基准值。</li>
<li><span class="arithmatex">\(a_1 x\)</span> 项增加斜率。</li>
<li><span class="arithmatex">\(a_2 x^2\)</span> 项增加曲率。</li>
<li>更高的幂次则捕捉函数形状的更多细节。</li>
</ul>
</li>
</ul>
<p><img alt="每个多项式项为逼近增加一层细节" src="../../images/polynomial_buildup.svg" /></p>
<ul>
<li>
<p>通过选择合适的系数,我们可以逐次匹配函数在某一点的值、斜率、曲率以及高阶行为。</p>
</li>
<li>
<p>当项数足够时,多项式几乎可以模仿任何光滑函数。</p>
</li>
<li>
<p>问题在于:如何找到正确的系数?</p>
</li>
<li>
<p><strong>线性化</strong>是最简单的逼近。在点 <span class="arithmatex">\(x = a\)</span> 附近,我们用函数的切线来代替它:</p>
</li>
</ul>
<div class="arithmatex">\[L(x) = f(a) + f'(a)(x - a)\]</div>
<ul>
<li>
<p>这是一阶<strong>泰勒逼近</strong>。它的思路是:从已知值 <span class="arithmatex">\(f(a)\)</span> 出发,然后加上斜率乘以距离 <span class="arithmatex">\(a\)</span> 的偏移量。</p>
</li>
<li>
<p>例如,在 <span class="arithmatex">\(x = 0\)</span> 处对 <span class="arithmatex">\(\sin(x)\)</span> 线性化:<span class="arithmatex">\(f(0) = 0\)</span><span class="arithmatex">\(f'(0) = \cos(0) = 1\)</span>,所以 <span class="arithmatex">\(L(x) = x\)</span>。在零附近,<span class="arithmatex">\(\sin(x) \approx x\)</span>。试试看:<span class="arithmatex">\(\sin(0.1) = 0.0998\ldots \approx 0.1\)</span></p>
</li>
<li>
<p>但线性化仅在非常接近 <span class="arithmatex">\(a\)</span> 的地方有效。离得稍远,逼近就失效了。为了做得更好,我们需要引入高阶项。</p>
</li>
<li>
<p><strong>泰勒级数</strong>将函数表示为无穷多个多项式项的和,每一项都捕捉到函数在点 <span class="arithmatex">\(a\)</span> 附近行为的更精细细节:</p>
</li>
</ul>
<div class="arithmatex">\[f(x) = \sum_{n=0}^{\infty} \frac{f^{(n)}(a)}{n!}(x - a)^n = f(a) + f'(a)(x-a) + \frac{f''(a)}{2!}(x-a)^2 + \frac{f'''(a)}{3!}(x-a)^3 + \cdots\]</div>
<p><img alt="泰勒级数:项数越多,逼近越精确" src="../../images/taylor_approximation.svg" /></p>
<ul>
<li>
<p>每一项依次增加一个修正项。第一项匹配函数值,第二项匹配斜率,第三项匹配曲率,依此类推。包含的项越多,逼近精确的区域就越大。</p>
</li>
<li>
<p>分母中的 <span class="arithmatex">\(n!\)</span> 并非随意选择。当你对 <span class="arithmatex">\((x - a)^n\)</span> 恰好微分 <span class="arithmatex">\(n\)</span> 次时,会得到 <span class="arithmatex">\(n!\)</span>。阶乘抵消了这个结果,从而确保泰勒多项式的 <span class="arithmatex">\(n\)</span> 阶导数在 <span class="arithmatex">\(x = a\)</span> 处与原函数的 <span class="arithmatex">\(n\)</span> 阶导数相等。</p>
</li>
<li>
<p><strong>麦克劳林级数</strong>就是中心在 <span class="arithmatex">\(a = 0\)</span> 的泰勒级数:</p>
</li>
</ul>
<div class="arithmatex">\[f(x) = \sum_{n=0}^{\infty} \frac{f^{(n)}(0)}{n!} x^n\]</div>
<ul>
<li>一些著名的麦克劳林级数:</li>
</ul>
<div class="arithmatex">\[e^x = 1 + x + \frac{x^2}{2!} + \frac{x^3}{3!} + \cdots\]</div>
<div class="arithmatex">\[\sin x = x - \frac{x^3}{3!} + \frac{x^5}{5!} - \frac{x^7}{7!} + \cdots\]</div>
<div class="arithmatex">\[\cos x = 1 - \frac{x^2}{2!} + \frac{x^4}{4!} - \frac{x^6}{6!} + \cdots\]</div>
<ul>
<li>
<p>注意 <span class="arithmatex">\(\sin x\)</span> 只有奇次幂(它是奇函数),而 <span class="arithmatex">\(\cos x\)</span> 只有偶次幂(它是偶函数)。交替的符号使得逼近在真实值周围振荡,从两侧同时收敛。</p>
</li>
<li>
<p>让我们用四项来逼近 <span class="arithmatex">\(e^{0.5}\)</span><span class="arithmatex">\(1 + 0.5 + \frac{0.25}{2} + \frac{0.125}{6} = 1 + 0.5 + 0.125 + 0.02083 \approx 1.6458\)</span>。真实值为 <span class="arithmatex">\(1.6487\ldots\)</span>,因此四项已经给出了三个正确的小数位。</p>
</li>
<li>
<p>并非所有泰勒级数都处处收敛。<strong>收敛半径</strong>告诉我们,在距离中心 <span class="arithmatex">\(a\)</span> 多远的范围内,级数给出有效的结果。在此半径内,通过增加项数,多项式逼近可以达到任意所需的精度。超出此半径,级数发散。</p>
</li>
<li>
<p><strong>幂级数</strong>的一般形式是:<span class="arithmatex">\(\sum_{n=0}^{\infty} a_n (x - c)^n\)</span>。泰勒级数是系数由导数确定的幂级数。其他幂级数可能由其他规则定义。<strong>比值判别法</strong>用于判定收敛性:计算 <span class="arithmatex">\(\lim_{n \to \infty} \left|\frac{a_{n+1}}{a_n}\right|\)</span>。如果该极限为 <span class="arithmatex">\(L\)</span>,则收敛半径为 <span class="arithmatex">\(R = 1/L\)</span></p>
</li>
<li>
<p>将泰勒级数截断到 <span class="arithmatex">\(n\)</span> 项时,会产生误差。<strong>拉格朗日余项</strong>给出了这个误差的界限:</p>
</li>
</ul>
<div class="arithmatex">\[R_n(x) = \frac{f^{(n+1)}(c)}{(n+1)!}(x-a)^{n+1}\]</div>
<ul>
<li>
<p>这里 <span class="arithmatex">\(c\)</span><span class="arithmatex">\(a\)</span><span class="arithmatex">\(x\)</span> 之间的某个未知点。我们无法确切知道 <span class="arithmatex">\(c\)</span>,但通常可以限定 <span class="arithmatex">\(|f^{(n+1)}(c)|\)</span> 来得到最坏情况下的误差估计。分母中的 <span class="arithmatex">\((n+1)!\)</span> 增长极快,因此随着项数增加,误差迅速减小(对于收敛半径内的函数而言)。</p>
</li>
<li>
<p>对于多变量函数,泰勒展开包含混合偏导数。<span class="arithmatex">\(f(\mathbf{x})\)</span> 在点 <span class="arithmatex">\(\mathbf{a}\)</span> 附近的二阶逼近为:</p>
</li>
</ul>
<div class="arithmatex">\[f(\mathbf{x}) \approx f(\mathbf{a}) + \nabla f(\mathbf{a})^T (\mathbf{x} - \mathbf{a}) + \frac{1}{2} (\mathbf{x} - \mathbf{a})^T H(\mathbf{a}) (\mathbf{x} - \mathbf{a})\]</div>
<ul>
<li>
<p>第一项是函数值,第二项使用梯度(向量,如我们在多元微积分中看到的),第三项使用海森矩阵(捕捉曲率)。这直接将我们的矩阵章节与微积分联系起来:海森矩阵是一个由二阶导数组成的矩阵,描述了函数表面的形状。</p>
</li>
<li>
<p>这种多变量二阶逼近是牛顿法和其他二阶优化技术的基础,我们将在下一个文件中看到。</p>
</li>
<li>
<p>除了多项式,还有其他值得了解的逼近方法:</p>
<ul>
<li><strong>样条插值</strong>:不用单个高次多项式,而是将多个低次多项式光滑拼接在一起。这避免了高次多项式可能产生的剧烈振荡。</li>
<li><strong>傅里叶级数</strong>:将周期函数逼近为正弦和余弦的和。在信号处理和音频中至关重要。</li>
<li><strong>神经网络</strong>:通用函数逼近器。只要有足够的神经元,它们可以任意精度逼近任何连续函数。这就是深度学习的理论基础。</li>
</ul>
</li>
<li>
<p>如果一个函数具有使逼近可靠的性质——连续性(无跳跃)、可微性(无尖角)、光滑性(所有阶导数都存在)和有界性(输出保持有限),我们就称其为"行为良好"的函数。</p>
</li>
<li>
<p>多项式、指数函数和三角函数都属于行为良好的函数。函数行为越好,获得良好逼近所需的泰勒项数就越少。</p>
</li>
</ul>
<h2 id="colab-jupyter-notebook">编程练习(使用 CoLab 或 Jupyter Notebook<a class="headerlink" href="#colab-jupyter-notebook" title="Permanent link">&para;</a></h2>
<ol>
<li>
<p>用递增数量的泰勒项逼近 <span class="arithmatex">\(e^x\)</span>,并可视化逼近效果如何改善。
<div class="highlight"><pre><span></span><code><a id="__codelineno-0-1" name="__codelineno-0-1" href="#__codelineno-0-1"></a><span class="kn">import</span><span class="w"> </span><span class="nn">jax.numpy</span><span class="w"> </span><span class="k">as</span><span class="w"> </span><span class="nn">jnp</span>
<a id="__codelineno-0-2" name="__codelineno-0-2" href="#__codelineno-0-2"></a><span class="kn">import</span><span class="w"> </span><span class="nn">matplotlib.pyplot</span><span class="w"> </span><span class="k">as</span><span class="w"> </span><span class="nn">plt</span>
<a id="__codelineno-0-3" name="__codelineno-0-3" href="#__codelineno-0-3"></a>
<a id="__codelineno-0-4" name="__codelineno-0-4" href="#__codelineno-0-4"></a><span class="n">x</span> <span class="o">=</span> <span class="n">jnp</span><span class="o">.</span><span class="n">linspace</span><span class="p">(</span><span class="o">-</span><span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">300</span><span class="p">)</span>
<a id="__codelineno-0-5" name="__codelineno-0-5" href="#__codelineno-0-5"></a><span class="n">plt</span><span class="o">.</span><span class="n">plot</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">jnp</span><span class="o">.</span><span class="n">exp</span><span class="p">(</span><span class="n">x</span><span class="p">),</span> <span class="s2">&quot;k-&quot;</span><span class="p">,</span> <span class="n">linewidth</span><span class="o">=</span><span class="mi">2</span><span class="p">,</span> <span class="n">label</span><span class="o">=</span><span class="s2">&quot;eˣ (精确值)&quot;</span><span class="p">)</span>
<a id="__codelineno-0-6" name="__codelineno-0-6" href="#__codelineno-0-6"></a>
<a id="__codelineno-0-7" name="__codelineno-0-7" href="#__codelineno-0-7"></a><span class="n">colors</span> <span class="o">=</span> <span class="p">[</span><span class="s2">&quot;#e74c3c&quot;</span><span class="p">,</span> <span class="s2">&quot;#3498db&quot;</span><span class="p">,</span> <span class="s2">&quot;#27ae60&quot;</span><span class="p">,</span> <span class="s2">&quot;#9b59b6&quot;</span><span class="p">]</span>
<a id="__codelineno-0-8" name="__codelineno-0-8" href="#__codelineno-0-8"></a><span class="k">for</span> <span class="n">n</span><span class="p">,</span> <span class="n">color</span> <span class="ow">in</span> <span class="nb">zip</span><span class="p">([</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">8</span><span class="p">],</span> <span class="n">colors</span><span class="p">):</span>
<a id="__codelineno-0-9" name="__codelineno-0-9" href="#__codelineno-0-9"></a> <span class="n">approx</span> <span class="o">=</span> <span class="nb">sum</span><span class="p">(</span><span class="n">x</span><span class="o">**</span><span class="n">k</span> <span class="o">/</span> <span class="n">jnp</span><span class="o">.</span><span class="n">array</span><span class="p">(</span><span class="nb">float</span><span class="p">(</span><span class="n">jnp</span><span class="o">.</span><span class="n">prod</span><span class="p">(</span><span class="n">jnp</span><span class="o">.</span><span class="n">arange</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="n">k</span><span class="o">+</span><span class="mi">1</span><span class="p">))</span> <span class="k">if</span> <span class="n">k</span> <span class="o">&gt;</span> <span class="mi">0</span> <span class="k">else</span> <span class="mi">1</span><span class="p">))</span>
<a id="__codelineno-0-10" name="__codelineno-0-10" href="#__codelineno-0-10"></a> <span class="k">for</span> <span class="n">k</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="n">n</span><span class="o">+</span><span class="mi">1</span><span class="p">))</span>
<a id="__codelineno-0-11" name="__codelineno-0-11" href="#__codelineno-0-11"></a> <span class="n">plt</span><span class="o">.</span><span class="n">plot</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">approx</span><span class="p">,</span> <span class="n">color</span><span class="o">=</span><span class="n">color</span><span class="p">,</span> <span class="n">linestyle</span><span class="o">=</span><span class="s2">&quot;--&quot;</span><span class="p">,</span> <span class="n">label</span><span class="o">=</span><span class="sa">f</span><span class="s2">&quot;</span><span class="si">{</span><span class="n">n</span><span class="si">}</span><span class="s2">&quot;</span><span class="p">)</span>
<a id="__codelineno-0-12" name="__codelineno-0-12" href="#__codelineno-0-12"></a>
<a id="__codelineno-0-13" name="__codelineno-0-13" href="#__codelineno-0-13"></a><span class="n">plt</span><span class="o">.</span><span class="n">ylim</span><span class="p">(</span><span class="o">-</span><span class="mi">2</span><span class="p">,</span> <span class="mi">15</span><span class="p">)</span>
<a id="__codelineno-0-14" name="__codelineno-0-14" href="#__codelineno-0-14"></a><span class="n">plt</span><span class="o">.</span><span class="n">legend</span><span class="p">()</span>
<a id="__codelineno-0-15" name="__codelineno-0-15" href="#__codelineno-0-15"></a><span class="n">plt</span><span class="o">.</span><span class="n">title</span><span class="p">(</span><span class="s2">&quot;eˣ 的泰勒逼近&quot;</span><span class="p">)</span>
<a id="__codelineno-0-16" name="__codelineno-0-16" href="#__codelineno-0-16"></a><span class="n">plt</span><span class="o">.</span><span class="n">show</span><span class="p">()</span>
</code></pre></div></p>
</li>
<li>
<p>计算拉格朗日余项,以限定用不同数量的泰勒项逼近 <span class="arithmatex">\(\sin(1)\)</span> 时的误差。
<div class="highlight"><pre><span></span><code><a id="__codelineno-1-1" name="__codelineno-1-1" href="#__codelineno-1-1"></a><span class="kn">import</span><span class="w"> </span><span class="nn">jax.numpy</span><span class="w"> </span><span class="k">as</span><span class="w"> </span><span class="nn">jnp</span>
<a id="__codelineno-1-2" name="__codelineno-1-2" href="#__codelineno-1-2"></a>
<a id="__codelineno-1-3" name="__codelineno-1-3" href="#__codelineno-1-3"></a><span class="n">x</span> <span class="o">=</span> <span class="mf">1.0</span>
<a id="__codelineno-1-4" name="__codelineno-1-4" href="#__codelineno-1-4"></a><span class="n">exact</span> <span class="o">=</span> <span class="n">jnp</span><span class="o">.</span><span class="n">sin</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
<a id="__codelineno-1-5" name="__codelineno-1-5" href="#__codelineno-1-5"></a>
<a id="__codelineno-1-6" name="__codelineno-1-6" href="#__codelineno-1-6"></a><span class="n">taylor</span> <span class="o">=</span> <span class="mf">0.0</span>
<a id="__codelineno-1-7" name="__codelineno-1-7" href="#__codelineno-1-7"></a><span class="k">for</span> <span class="n">n</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">8</span><span class="p">):</span>
<a id="__codelineno-1-8" name="__codelineno-1-8" href="#__codelineno-1-8"></a> <span class="n">sign</span> <span class="o">=</span> <span class="p">(</span><span class="o">-</span><span class="mi">1</span><span class="p">)</span><span class="o">**</span><span class="n">n</span>
<a id="__codelineno-1-9" name="__codelineno-1-9" href="#__codelineno-1-9"></a> <span class="n">factorial</span> <span class="o">=</span> <span class="nb">float</span><span class="p">(</span><span class="n">jnp</span><span class="o">.</span><span class="n">prod</span><span class="p">(</span><span class="n">jnp</span><span class="o">.</span><span class="n">arange</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="o">*</span><span class="n">n</span><span class="o">+</span><span class="mi">2</span><span class="p">)))</span>
<a id="__codelineno-1-10" name="__codelineno-1-10" href="#__codelineno-1-10"></a> <span class="n">taylor</span> <span class="o">+=</span> <span class="n">sign</span> <span class="o">*</span> <span class="n">x</span><span class="o">**</span><span class="p">(</span><span class="mi">2</span><span class="o">*</span><span class="n">n</span><span class="o">+</span><span class="mi">1</span><span class="p">)</span> <span class="o">/</span> <span class="n">factorial</span>
<a id="__codelineno-1-11" name="__codelineno-1-11" href="#__codelineno-1-11"></a> <span class="n">error</span> <span class="o">=</span> <span class="nb">abs</span><span class="p">(</span><span class="n">exact</span> <span class="o">-</span> <span class="n">taylor</span><span class="p">)</span>
<a id="__codelineno-1-12" name="__codelineno-1-12" href="#__codelineno-1-12"></a> <span class="n">bound</span> <span class="o">=</span> <span class="n">x</span><span class="o">**</span><span class="p">(</span><span class="mi">2</span><span class="o">*</span><span class="n">n</span><span class="o">+</span><span class="mi">3</span><span class="p">)</span> <span class="o">/</span> <span class="nb">float</span><span class="p">(</span><span class="n">jnp</span><span class="o">.</span><span class="n">prod</span><span class="p">(</span><span class="n">jnp</span><span class="o">.</span><span class="n">arange</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="o">*</span><span class="n">n</span><span class="o">+</span><span class="mi">4</span><span class="p">)))</span>
<a id="__codelineno-1-13" name="__codelineno-1-13" href="#__codelineno-1-13"></a> <span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;项数=</span><span class="si">{</span><span class="n">n</span><span class="o">+</span><span class="mi">1</span><span class="si">}</span><span class="s2"> 近似值=</span><span class="si">{</span><span class="n">taylor</span><span class="si">:</span><span class="s2">.10f</span><span class="si">}</span><span class="s2"> 误差=</span><span class="si">{</span><span class="n">error</span><span class="si">:</span><span class="s2">.2e</span><span class="si">}</span><span class="s2"> 界限=</span><span class="si">{</span><span class="n">bound</span><span class="si">:</span><span class="s2">.2e</span><span class="si">}</span><span class="s2">&quot;</span><span class="p">)</span>
</code></pre></div></p>
</li>
<li>
<p>比较在 <span class="arithmatex">\(x=0\)</span> 附近 <span class="arithmatex">\(\cos(x)\)</span> 的线性化逼近与二次泰勒逼近。在同一张图上绘制两个逼近和真实函数,观察各自精确的范围。
<div class="highlight"><pre><span></span><code><a id="__codelineno-2-1" name="__codelineno-2-1" href="#__codelineno-2-1"></a><span class="kn">import</span><span class="w"> </span><span class="nn">jax.numpy</span><span class="w"> </span><span class="k">as</span><span class="w"> </span><span class="nn">jnp</span>
<a id="__codelineno-2-2" name="__codelineno-2-2" href="#__codelineno-2-2"></a><span class="kn">import</span><span class="w"> </span><span class="nn">matplotlib.pyplot</span><span class="w"> </span><span class="k">as</span><span class="w"> </span><span class="nn">plt</span>
<a id="__codelineno-2-3" name="__codelineno-2-3" href="#__codelineno-2-3"></a>
<a id="__codelineno-2-4" name="__codelineno-2-4" href="#__codelineno-2-4"></a><span class="n">x</span> <span class="o">=</span> <span class="n">jnp</span><span class="o">.</span><span class="n">linspace</span><span class="p">(</span><span class="o">-</span><span class="mi">3</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">300</span><span class="p">)</span>
<a id="__codelineno-2-5" name="__codelineno-2-5" href="#__codelineno-2-5"></a><span class="n">plt</span><span class="o">.</span><span class="n">plot</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">jnp</span><span class="o">.</span><span class="n">cos</span><span class="p">(</span><span class="n">x</span><span class="p">),</span> <span class="s2">&quot;k-&quot;</span><span class="p">,</span> <span class="n">linewidth</span><span class="o">=</span><span class="mi">2</span><span class="p">,</span> <span class="n">label</span><span class="o">=</span><span class="s2">&quot;cos(x)&quot;</span><span class="p">)</span>
<a id="__codelineno-2-6" name="__codelineno-2-6" href="#__codelineno-2-6"></a><span class="n">plt</span><span class="o">.</span><span class="n">plot</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">jnp</span><span class="o">.</span><span class="n">ones_like</span><span class="p">(</span><span class="n">x</span><span class="p">),</span> <span class="s2">&quot;--&quot;</span><span class="p">,</span> <span class="n">color</span><span class="o">=</span><span class="s2">&quot;#e74c3c&quot;</span><span class="p">,</span> <span class="n">label</span><span class="o">=</span><span class="s2">&quot;线性: 1&quot;</span><span class="p">)</span>
<a id="__codelineno-2-7" name="__codelineno-2-7" href="#__codelineno-2-7"></a><span class="n">plt</span><span class="o">.</span><span class="n">plot</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="mi">1</span> <span class="o">-</span> <span class="n">x</span><span class="o">**</span><span class="mi">2</span><span class="o">/</span><span class="mi">2</span><span class="p">,</span> <span class="s2">&quot;--&quot;</span><span class="p">,</span> <span class="n">color</span><span class="o">=</span><span class="s2">&quot;#3498db&quot;</span><span class="p">,</span> <span class="n">label</span><span class="o">=</span><span class="s2">&quot;二次: 1 - x²/2&quot;</span><span class="p">)</span>
<a id="__codelineno-2-8" name="__codelineno-2-8" href="#__codelineno-2-8"></a><span class="n">plt</span><span class="o">.</span><span class="n">plot</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="mi">1</span> <span class="o">-</span> <span class="n">x</span><span class="o">**</span><span class="mi">2</span><span class="o">/</span><span class="mi">2</span> <span class="o">+</span> <span class="n">x</span><span class="o">**</span><span class="mi">4</span><span class="o">/</span><span class="mi">24</span><span class="p">,</span> <span class="s2">&quot;--&quot;</span><span class="p">,</span> <span class="n">color</span><span class="o">=</span><span class="s2">&quot;#27ae60&quot;</span><span class="p">,</span> <span class="n">label</span><span class="o">=</span><span class="s2">&quot;四阶&quot;</span><span class="p">)</span>
<a id="__codelineno-2-9" name="__codelineno-2-9" href="#__codelineno-2-9"></a><span class="n">plt</span><span class="o">.</span><span class="n">ylim</span><span class="p">(</span><span class="o">-</span><span class="mi">2</span><span class="p">,</span> <span class="mi">2</span><span class="p">)</span>
<a id="__codelineno-2-10" name="__codelineno-2-10" href="#__codelineno-2-10"></a><span class="n">plt</span><span class="o">.</span><span class="n">legend</span><span class="p">()</span>
<a id="__codelineno-2-11" name="__codelineno-2-11" href="#__codelineno-2-11"></a><span class="n">plt</span><span class="o">.</span><span class="n">title</span><span class="p">(</span><span class="s2">&quot;cos(x) 的泰勒逼近&quot;</span><span class="p">)</span>
<a id="__codelineno-2-12" name="__codelineno-2-12" href="#__codelineno-2-12"></a><span class="n">plt</span><span class="o">.</span><span class="n">show</span><span class="p">()</span>
</code></pre></div></p>
</li>
</ol>
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