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<article class="md-content__inner md-typeset">
<h1 id="_1">优化<a class="headerlink" href="#_1" title="Permanent link">&para;</a></h1>
<p><em>优化是模型训练的数学核心——寻找使损失函数最小的参数。本文涵盖驻点、凸性、梯度下降、牛顿法、带拉格朗日乘数的约束优化,以及驱动现代深度学习的主流优化器(SGD、Adam)。</em></p>
<ul>
<li>
<p>训练神经网络、拟合回归线、调优超参数:几乎所有机器学习算法的核心都是一个<strong>优化</strong>问题。</p>
</li>
<li>
<p>我们有一个函数(损失函数、代价函数、目标函数),希望找到使其尽可能小(或大)的输入。</p>
</li>
<li>
<p>在优化之前,我们需要理解函数的<strong>零点</strong>(或根)。<span class="arithmatex">\(f(x)\)</span> 的零点是指满足 <span class="arithmatex">\(f(x) = 0\)</span><span class="arithmatex">\(x\)</span> 值。从图形上看,这些点就是与 x 轴的交点。</p>
</li>
<li>
<p>例如,<span class="arithmatex">\(f(x) = x^2 - 3x + 2 = (x-1)(x-2)\)</span> 的零点在 <span class="arithmatex">\(x = 1\)</span><span class="arithmatex">\(x = 2\)</span> 处。在两个零点之间,函数为负(<span class="arithmatex">\(f(1.5) = -0.25\)</span>);在零点之外,函数为正。零点将数轴分割成若干个区域,在每个区域中函数保持相同符号。</p>
</li>
<li>
<p>零点的<strong>重数</strong>是指对应因式出现的次数。</p>
</li>
<li>
<p>在单零点(重数为 1)处,图像穿过 x 轴。在二重零点(重数为 2)处,图像接触 x 轴但反弹回去而不穿过,在该点处看起来是"平坦"的。</p>
</li>
<li>
<p>寻找零点之所以重要,是因为导数 <span class="arithmatex">\(f'(x)\)</span> 的零点正是 <span class="arithmatex">\(f(x)\)</span><strong>驻点</strong>——即极大值或极小值的候选点。</p>
</li>
<li>
<p>在极大值或极小值处,切线是水平的(斜率为 0),因此 <span class="arithmatex">\(f'(x) = 0\)</span></p>
</li>
</ul>
<p><img alt="驻点:当导数为零时,函数出现波峰、波谷或鞍点" src="../../images/critical_points.svg" /></p>
<ul>
<li>
<p>但并非每个驻点都是极大值或极小值。<span class="arithmatex">\(f'(x) = 0\)</span> 的点也可能是<strong>拐点</strong>(如 <span class="arithmatex">\(f(x) = x^3\)</span><span class="arithmatex">\(x = 0\)</span> 处),函数在该点暂时变平但并未改变方向。</p>
</li>
<li>
<p><strong>二阶导数检验</strong>可以解决这个问题。在驻点 <span class="arithmatex">\(x = c\)</span>(即 <span class="arithmatex">\(f'(c) = 0\)</span>)处:</p>
<ul>
<li><span class="arithmatex">\(f''(c) &gt; 0\)</span>:曲线向下凸(碗状),因此 <span class="arithmatex">\(c\)</span><strong>局部极小值</strong></li>
<li><span class="arithmatex">\(f''(c) &lt; 0\)</span>:曲线向上凸(山丘状),因此 <span class="arithmatex">\(c\)</span><strong>局部极大值</strong></li>
<li><span class="arithmatex">\(f''(c) = 0\)</span>:检验无效,需要使用更高阶导数或其他方法。</li>
</ul>
</li>
<li>
<p>例如,<span class="arithmatex">\(f(x) = x^3 - 3x\)</span>。导数为 <span class="arithmatex">\(f'(x) = 3x^2 - 3 = 3(x-1)(x+1)\)</span>,因此驻点在 <span class="arithmatex">\(x = -1\)</span><span class="arithmatex">\(x = 1\)</span> 处。二阶导数为 <span class="arithmatex">\(f''(x) = 6x\)</span>。在 <span class="arithmatex">\(x = -1\)</span> 处:<span class="arithmatex">\(f''(-1) = -6 &lt; 0\)</span>(局部极大值)。在 <span class="arithmatex">\(x = 1\)</span> 处:<span class="arithmatex">\(f''(1) = 6 &gt; 0\)</span>(局部极小值)。</p>
</li>
<li>
<p>如果连接函数图像上任意两点的线段位于图像之上(或与之重合),则该函数是<strong>凸的</strong>。可以想象成一个碗形,处处向上弯曲。数学上,若对所有 <span class="arithmatex">\(x\)</span><span class="arithmatex">\(f''(x) \geq 0\)</span>,则 <span class="arithmatex">\(f\)</span> 是凸函数。</p>
</li>
</ul>
<p><img alt="凸函数具有唯一的全局最小值;非凸函数可能有多个局部最小值" src="../../images/convex_nonconvex.svg" /></p>
<ul>
<li>
<p>凸性的强大之处在于凸函数有一个卓越的性质:每个局部极小值同时也是<strong>全局最小值</strong>。不存在会让人陷入的欺骗性局部低谷。如果你把一个球滚入凸碗中,它总是会到达底部。</p>
</li>
<li>
<p><span class="arithmatex">\(-f\)</span> 是凸的,则函数是<strong>凹的</strong>(向下弯曲)。函数从凹性过渡到凸性的点称为<strong>拐点</strong>,出现在 <span class="arithmatex">\(f''(x) = 0\)</span> 处。</p>
</li>
<li>
<p><strong>牛顿法</strong>利用切线寻找函数的零点(进而也可用于寻找其导数的驻点)。从初始猜测 <span class="arithmatex">\(x_0\)</span> 出发,迭代更新:</p>
</li>
</ul>
<div class="arithmatex">\[x_{n+1} = x_n - \frac{f(x_n)}{f'(x_n)}\]</div>
<p><img alt="牛顿法:沿切线方向逼近根的更好近似值" src="../../images/newtons_method.svg" /></p>
<ul>
<li>
<p>其思想是:在 <span class="arithmatex">\(x_n\)</span> 处画出切线,找到它与 x 轴的交点,该交点即为 <span class="arithmatex">\(x_{n+1}\)</span>。对于性质良好且初始点选取恰当的函数,牛顿法收敛非常快(二次收敛,即每步正确位数大致翻倍)。</p>
</li>
<li>
<p>例如,求 <span class="arithmatex">\(\sqrt{5}\)</span>(即 <span class="arithmatex">\(f(x) = x^2 - 5\)</span> 的零点):<span class="arithmatex">\(f'(x) = 2x\)</span>,因此 <span class="arithmatex">\(x_{n+1} = x_n - \frac{x_n^2 - 5}{2x_n}\)</span>。从 <span class="arithmatex">\(x_0 = 2\)</span> 开始:<span class="arithmatex">\(x_1 = 2.25\)</span><span class="arithmatex">\(x_2 = 2.2361\ldots\)</span>,已精确到小数点后四位。</p>
</li>
<li>
<p>如果初始猜测离根太远、根附近 <span class="arithmatex">\(f'(x) = 0\)</span>,或函数在附近有拐点,牛顿法可能会失败。此外,它还需要计算导数,这可能代价高昂。</p>
</li>
<li>
<p>对于优化(寻找极小值而非零点),我们将牛顿法应用于 <span class="arithmatex">\(f'(x) = 0\)</span>,得到更新公式:</p>
</li>
</ul>
<div class="arithmatex">\[x_{n+1} = x_n - \frac{f'(x_n)}{f''(x_n)}\]</div>
<ul>
<li>
<p>在多维情形下,这变为 <span class="arithmatex">\(\mathbf{x}_{n+1} = \mathbf{x}_n - H^{-1} \nabla f(\mathbf{x}_n)\)</span>,其中 <span class="arithmatex">\(H\)</span> 是 Hessian 矩阵。这正是上一节中二阶泰勒近似的实际应用:将函数近似为二次型,跳到该二次型的极小值点,然后重复。</p>
</li>
<li>
<p><strong>拉格朗日乘数</strong>用于求解<strong>约束优化</strong>:在约束条件 <span class="arithmatex">\(g(x, y) = c\)</span> 下求 <span class="arithmatex">\(f(x, y)\)</span> 的最优值。我们不是在 <span class="arithmatex">\(\mathbb{R}^n\)</span> 中全域搜索,而是限制在约束条件成立的集合(一条曲线或曲面)上。</p>
</li>
<li>
<p>关键见解是几何层面的:在约束最优解处,<span class="arithmatex">\(f\)</span> 的梯度必须与 <span class="arithmatex">\(g\)</span> 的梯度平行。如果它们不平行,我们可以沿着约束条件朝某个方向移动,从而继续改进 <span class="arithmatex">\(f\)</span> 的值,这意味着还没有达到最优。</p>
</li>
<li>
<p>我们引入一个新变量 <span class="arithmatex">\(\lambda\)</span>(拉格朗日乘数),定义<strong>拉格朗日函数</strong></p>
</li>
</ul>
<div class="arithmatex">\[\mathcal{L}(x, y, \lambda) = f(x, y) - \lambda(g(x, y) - c)\]</div>
<ul>
<li>令所有偏导数为零,得到一个方程组,其解即为约束最优解:</li>
</ul>
<div class="arithmatex">\[\frac{\partial \mathcal{L}}{\partial x} = 0, \quad \frac{\partial \mathcal{L}}{\partial y} = 0, \quad \frac{\partial \mathcal{L}}{\partial \lambda} = 0\]</div>
<p><img alt="拉格朗日乘数:在最优解处,f 和 g 的梯度平行" src="../../images/lagrange_multiplier.svg" /></p>
<ul>
<li>例如,在 <span class="arithmatex">\(x^2 + y^2 = 1\)</span> 的约束下最大化 <span class="arithmatex">\(f(x,y) = x^2 y\)</span>。拉格朗日函数为 <span class="arithmatex">\(\mathcal{L} = x^2 y - \lambda(x^2 + y^2 - 1)\)</span>。求偏导:</li>
</ul>
<div class="arithmatex">\[2xy - 2\lambda x = 0, \quad x^2 - 2\lambda y = 0, \quad x^2 + y^2 = 1\]</div>
<ul>
<li>
<p>由第一个方程(假设 <span class="arithmatex">\(x \neq 0\)</span>):<span class="arithmatex">\(\lambda = y\)</span>。代入第二个方程:<span class="arithmatex">\(x^2 = 2y^2\)</span>。结合约束条件:<span class="arithmatex">\(2y^2 + y^2 = 1\)</span>,得 <span class="arithmatex">\(y = \frac{1}{\sqrt{3}}\)</span>。最大值为 <span class="arithmatex">\(f = \frac{2}{3\sqrt{3}}\)</span></p>
</li>
<li>
<p>对于不等式约束(<span class="arithmatex">\(g(x,y) \leq c\)</span> 而非 <span class="arithmatex">\(= c\)</span>),<strong>Karush-Kuhn-TuckerKKT)条件</strong>推广了拉格朗日乘数法。约束要么是激活的(有效约束,按等式处理),要么是非激活的(解在内部,约束无关紧要)。</p>
</li>
<li>
<p>在实践中,我们很少手工进行优化。以下是主要的算法家族:</p>
<ul>
<li>
<p><strong>一阶方法</strong>(仅使用梯度):梯度下降、随机梯度下降(SGD)、Adam。这些方法每步计算成本低,但收敛可能较慢,尤其是在病态问题上。</p>
</li>
<li>
<p><strong>二阶方法</strong>(使用梯度和 Hessian 矩阵):牛顿法收敛快,但计算和求逆 Hessian 矩阵代价高昂(对于 <span class="arithmatex">\(n\)</span> 个参数为 <span class="arithmatex">\(O(n^3)\)</span>)。<strong>拟牛顿法</strong>(如 BFGS 和 L-BFGS)仅利用梯度信息近似 Hessian 矩阵,比一阶方法收敛更快,又无需承担完全的二阶方法计算成本。</p>
</li>
<li>
<p><strong>共轭梯度法</strong>:适用于大型稀疏系统,仅需矩阵-向量乘积,无需存储完整的 Hessian 矩阵。</p>
</li>
<li>
<p><strong>高斯-牛顿法</strong><strong>莱文贝格-马夸尔特法</strong>:专门用于最小二乘问题(在回归中常见),通过 Jacobian 矩阵近似 Hessian 矩阵。</p>
</li>
<li>
<p><strong>自然梯度下降</strong>:利用 Fisher 信息矩阵考虑参数空间的几何结构,对概率模型可能更有效。</p>
</li>
</ul>
</li>
<li>
<p>优化器的选择取决于具体问题。对于深度学习,一阶方法(尤其是 Adam)占主导地位,因为参数量巨大(数百万到数十亿),计算 Hessian 矩阵不切实际。对于目标函数光滑的小规模问题,二阶方法可能快得多。</p>
</li>
</ul>
<h2 id="colab-notebook">编程练习(在 CoLab 或 notebook 中完成)<a class="headerlink" href="#colab-notebook" title="Permanent link">&para;</a></h2>
<ol>
<li>
<p>实现牛顿法求 <span class="arithmatex">\(\sqrt{7}\)</span>(即 <span class="arithmatex">\(f(x) = x^2 - 7\)</span> 的零点)。观察其快速收敛。
<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>
<a id="__codelineno-0-3" name="__codelineno-0-3" href="#__codelineno-0-3"></a><span class="n">f</span> <span class="o">=</span> <span class="k">lambda</span> <span class="n">x</span><span class="p">:</span> <span class="n">x</span><span class="o">**</span><span class="mi">2</span> <span class="o">-</span> <span class="mi">7</span>
<a id="__codelineno-0-4" name="__codelineno-0-4" href="#__codelineno-0-4"></a><span class="n">df</span> <span class="o">=</span> <span class="k">lambda</span> <span class="n">x</span><span class="p">:</span> <span class="mi">2</span><span class="o">*</span><span class="n">x</span>
<a id="__codelineno-0-5" name="__codelineno-0-5" href="#__codelineno-0-5"></a>
<a id="__codelineno-0-6" name="__codelineno-0-6" href="#__codelineno-0-6"></a><span class="n">x</span> <span class="o">=</span> <span class="mf">3.0</span> <span class="c1"># 初始猜测</span>
<a id="__codelineno-0-7" name="__codelineno-0-7" href="#__codelineno-0-7"></a><span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">6</span><span class="p">):</span>
<a id="__codelineno-0-8" name="__codelineno-0-8" href="#__codelineno-0-8"></a> <span class="n">x</span> <span class="o">=</span> <span class="n">x</span> <span class="o">-</span> <span class="n">f</span><span class="p">(</span><span class="n">x</span><span class="p">)</span> <span class="o">/</span> <span class="n">df</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
<a id="__codelineno-0-9" name="__codelineno-0-9" href="#__codelineno-0-9"></a> <span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;step </span><span class="si">{</span><span class="n">i</span><span class="o">+</span><span class="mi">1</span><span class="si">}</span><span class="s2">: x = </span><span class="si">{</span><span class="n">x</span><span class="si">:</span><span class="s2">.10f</span><span class="si">}</span><span class="s2"> (error: </span><span class="si">{</span><span class="nb">abs</span><span class="p">(</span><span class="n">x</span><span class="w"> </span><span class="o">-</span><span class="w"> </span><span class="n">jnp</span><span class="o">.</span><span class="n">sqrt</span><span class="p">(</span><span class="mf">7.0</span><span class="p">))</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>使用梯度下降最小化 <span class="arithmatex">\(f(x, y) = (x - 3)^2 + (y + 1)^2\)</span>。最小值在 <span class="arithmatex">\((3, -1)\)</span> 处。尝试不同的学习率。
<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</span>
<a id="__codelineno-1-2" name="__codelineno-1-2" href="#__codelineno-1-2"></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-1-3" name="__codelineno-1-3" href="#__codelineno-1-3"></a>
<a id="__codelineno-1-4" name="__codelineno-1-4" href="#__codelineno-1-4"></a><span class="k">def</span><span class="w"> </span><span class="nf">f</span><span class="p">(</span><span class="n">params</span><span class="p">):</span>
<a id="__codelineno-1-5" name="__codelineno-1-5" href="#__codelineno-1-5"></a> <span class="n">x</span><span class="p">,</span> <span class="n">y</span> <span class="o">=</span> <span class="n">params</span>
<a id="__codelineno-1-6" name="__codelineno-1-6" href="#__codelineno-1-6"></a> <span class="k">return</span> <span class="p">(</span><span class="n">x</span> <span class="o">-</span> <span class="mi">3</span><span class="p">)</span><span class="o">**</span><span class="mi">2</span> <span class="o">+</span> <span class="p">(</span><span class="n">y</span> <span class="o">+</span> <span class="mi">1</span><span class="p">)</span><span class="o">**</span><span class="mi">2</span>
<a id="__codelineno-1-7" name="__codelineno-1-7" href="#__codelineno-1-7"></a>
<a id="__codelineno-1-8" name="__codelineno-1-8" href="#__codelineno-1-8"></a><span class="n">grad_f</span> <span class="o">=</span> <span class="n">jax</span><span class="o">.</span><span class="n">grad</span><span class="p">(</span><span class="n">f</span><span class="p">)</span>
<a id="__codelineno-1-9" name="__codelineno-1-9" href="#__codelineno-1-9"></a><span class="n">params</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="mf">0.0</span><span class="p">,</span> <span class="mf">0.0</span><span class="p">])</span>
<a id="__codelineno-1-10" name="__codelineno-1-10" href="#__codelineno-1-10"></a><span class="n">lr</span> <span class="o">=</span> <span class="mf">0.1</span>
<a id="__codelineno-1-11" name="__codelineno-1-11" href="#__codelineno-1-11"></a>
<a id="__codelineno-1-12" name="__codelineno-1-12" href="#__codelineno-1-12"></a><span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">20</span><span class="p">):</span>
<a id="__codelineno-1-13" name="__codelineno-1-13" href="#__codelineno-1-13"></a> <span class="n">g</span> <span class="o">=</span> <span class="n">grad_f</span><span class="p">(</span><span class="n">params</span><span class="p">)</span>
<a id="__codelineno-1-14" name="__codelineno-1-14" href="#__codelineno-1-14"></a> <span class="n">params</span> <span class="o">=</span> <span class="n">params</span> <span class="o">-</span> <span class="n">lr</span> <span class="o">*</span> <span class="n">g</span>
<a id="__codelineno-1-15" name="__codelineno-1-15" href="#__codelineno-1-15"></a> <span class="k">if</span> <span class="n">i</span> <span class="o">%</span> <span class="mi">5</span> <span class="o">==</span> <span class="mi">0</span> <span class="ow">or</span> <span class="n">i</span> <span class="o">==</span> <span class="mi">19</span><span class="p">:</span>
<a id="__codelineno-1-16" name="__codelineno-1-16" href="#__codelineno-1-16"></a> <span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;step </span><span class="si">{</span><span class="n">i</span><span class="si">:</span><span class="s2">2d</span><span class="si">}</span><span class="s2">: (</span><span class="si">{</span><span class="n">params</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span><span class="si">:</span><span class="s2">.4f</span><span class="si">}</span><span class="s2">, </span><span class="si">{</span><span class="n">params</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span><span class="si">:</span><span class="s2">.4f</span><span class="si">}</span><span class="s2">) loss=</span><span class="si">{</span><span class="n">f</span><span class="p">(</span><span class="n">params</span><span class="p">)</span><span class="si">:</span><span class="s2">.6f</span><span class="si">}</span><span class="s2">&quot;</span><span class="p">)</span>
</code></pre></div></p>
</li>
<li>
<p>数值求解约束优化问题。在 <span class="arithmatex">\(x + y = 10\)</span> 的约束下最大化 <span class="arithmatex">\(f(x,y) = xy\)</span>,通过参数化 <span class="arithmatex">\(y = 10 - x\)</span> 并求单变量函数的最优值。
<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</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">jax.numpy</span><span class="w"> </span><span class="k">as</span><span class="w"> </span><span class="nn">jnp</span>
<a id="__codelineno-2-3" 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="c1"># 代入约束条件:y = 10 - x,所以 f = x(10 - x) = 10x - x²</span>
<a id="__codelineno-2-5" name="__codelineno-2-5" href="#__codelineno-2-5"></a><span class="n">f</span> <span class="o">=</span> <span class="k">lambda</span> <span class="n">x</span><span class="p">:</span> <span class="n">x</span> <span class="o">*</span> <span class="p">(</span><span class="mi">10</span> <span class="o">-</span> <span class="n">x</span><span class="p">)</span>
<a id="__codelineno-2-6" name="__codelineno-2-6" href="#__codelineno-2-6"></a><span class="n">df</span> <span class="o">=</span> <span class="n">jax</span><span class="o">.</span><span class="n">grad</span><span class="p">(</span><span class="n">f</span><span class="p">)</span>
<a id="__codelineno-2-7" name="__codelineno-2-7" href="#__codelineno-2-7"></a>
<a id="__codelineno-2-8" name="__codelineno-2-8" href="#__codelineno-2-8"></a><span class="c1"># 梯度上升(我们要求最大值,所以加上梯度)</span>
<a id="__codelineno-2-9" name="__codelineno-2-9" href="#__codelineno-2-9"></a><span class="n">x</span> <span class="o">=</span> <span class="mf">1.0</span>
<a id="__codelineno-2-10" name="__codelineno-2-10" href="#__codelineno-2-10"></a><span class="n">lr</span> <span class="o">=</span> <span class="mf">0.1</span>
<a id="__codelineno-2-11" name="__codelineno-2-11" href="#__codelineno-2-11"></a><span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">20</span><span class="p">):</span>
<a id="__codelineno-2-12" name="__codelineno-2-12" href="#__codelineno-2-12"></a> <span class="n">x</span> <span class="o">=</span> <span class="n">x</span> <span class="o">+</span> <span class="n">lr</span> <span class="o">*</span> <span class="n">df</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
<a id="__codelineno-2-13" name="__codelineno-2-13" href="#__codelineno-2-13"></a><span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;x=</span><span class="si">{</span><span class="n">x</span><span class="si">:</span><span class="s2">.4f</span><span class="si">}</span><span class="s2">, y=</span><span class="si">{</span><span class="mi">10</span><span class="o">-</span><span class="n">x</span><span class="si">:</span><span class="s2">.4f</span><span class="si">}</span><span class="s2">, f=</span><span class="si">{</span><span class="n">f</span><span class="p">(</span><span class="n">x</span><span class="p">)</span><span class="si">:</span><span class="s2">.4f</span><span class="si">}</span><span class="s2">&quot;</span><span class="p">)</span> <span class="c1"># 应为 x=5, y=5, f=25</span>
</code></pre></div></p>
</li>
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