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                  <h1>数列不等式 `lhc@jinan|20200407`</h1>
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                        	<p>已知数列 \(\{a_n\}\) 满足 \(a_1 = 1\), \(a_n a_{n+1} = n\), \(n = 1,2,3,\dots\). 求证: <br/>
\[<br/>
\frac1{a_1} + \frac1{a_2} + \frac1{a_3} + \dots + \frac1{a_n} \ge 2 \sqrt n - 1.<br/>
\]</p>


                        
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                  <h1>二元有理函数最值</h1>
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                        	<p>已知 \(x,y &gt; 0\), 求 <br/>
\[<br/>
\frac{(2x+1) (y+1)}{2x^2 + 5y^2 + 7}<br/>
\]<br/>
的最大值.</p>


                        
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                  <h1>两个数列不等式问题</h1>
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                        	<ol>
<li>已知实数 \(x_1, x_2, \dots, x_{10}​\), \(\sum\limits_{k=1}^{10} k x_k = 1​\). 求 \(\left( \sum\limits_{k=1}^{10} x_k \right)^2 + \sum\limits_{k=1}^{10} x_k^2​\) 的最小值.</li>
<li>设 \(a_n = 1 + \dfrac12 + \dfrac13 + \dots + \dfrac1n\). 求证: 对 \(n\ge 2\) 有 \(a_n^2 &lt; 2 \left( \dfrac{a_2}2 + \dfrac{a_3}3 + \dots \dfrac{a_n}n \right) + \dfrac{n+3}{2n+2}\).</li>
</ol>


                        
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                  <h1>CMO 2018 题 3</h1>
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                        	<p><mark>TODO: 微博图床已挂, 本篇缺图, 需要找回</mark></p>

<p>\(\triangle ABC\) 中, \(AB &lt; AC\), \(O\) 为外心, \(D\) 是 \(\angle BAC\) 平分线上一点, \(E\) 在 \(BC\) 上, 满足 \(OE \parallel AD\), \(DE \perp BC\). 在射线 \(EB\) 上取点 \(K\) 满足 \(EK = EA\), \(\triangle ADK\) 外接圆与 \(BC\) 交于另一点 \(P \ne K\), \(\triangle ADK\) 外接圆与 \(\triangle ABC\) 外接圆交于另一点 \(Q \ne A\). 求证: \(PQ\) 与 \(\triangle ABC\) 外接圆相切.</p>


                        
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                  <h1>两个二次函数问题</h1>
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                        	<ol>
<li>已知二次函数 \(f(x) = a x^2 + b x + c\) 的图象过点 \((2,8)\), 且对一切实数 \(x\) 恒有 \(2x + 3 \le f(x) \le 2x^2 - 2x + 5\), 求 \(f(x)\).</li>
<li>在平面直角坐标系中, 不论 \(m\) 取何值时, 抛物线 \(y = mx^2 + (2m+1) x - (3m+2)\) 都不通过的直线 \(y = -x+1\) 上的点有哪些? (写出全部符合条件点的坐标)</li>
</ol>


                        
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                  <h1>伪装成函数方程的数论问题</h1>
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                        	<p>已知函数 \(f(x,y)\) 定义在正整数集上, 满足 \(\forall x,y \in \mathbb N^*\),</p>

<ol>
<li>\(f(x,x) = x\),</li>
<li>\(f(x,y) = f(y,x)\),</li>
<li>\((x+y) \cdot f(x,y) = y \cdot f(x, x+y)\).</li>
</ol>

<p>求证: \(f(x,y) = [x,y]\) (这里, \([x,y]\) 表示 \(x\) 与 \(y\) 的最小公倍数).</p>


                        
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                  <h1>二次根式化简</h1>
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                        	<p>计算:<br/>
\[<br/>
\dfrac{<br/>
1 + \sqrt{2-\sqrt2} + \sqrt{2-\sqrt3}<br/>
}{<br/>
\sqrt3 + \sqrt{2+\sqrt2} + \sqrt{2+\sqrt3}<br/>
}<br/>
\]</p>


                        
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                  <h1>liujihang 20180801</h1>
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                        	<p><mark>TODO: 微博图床已挂, 本篇缺图, 需要找回</mark></p>

<p>无穷数列 \(P \colon a_1, a_2, \dots, a_n, \dots\) 满足 \(a_i \in \mathbb N^*\), 且 \(a_i \le a_{i+1}\) (\(i\in \mathbb N^*\)). 对于数列 \(P\), 记 \(T_k (P) = \min \{ n | a_n \ge k \}\) (\(k\in \mathbb N^*\)), 其中 \(\min \{ n | a_n \ge k \}\) 表示集合 \(\{ n | a_n \ge k \}\) 中最小的数.</p>

<ol>
<li>若数列 \(P\colon 1,3,4,7,\dots\), 写出 \(T_1(P), T_2(P), \dots, T_5(P)\);</li>
<li>若 \(T_k (P) = 2k-1\), 求数列 \(P\) 的前 \(n\) 项之和;</li>
<li>已知 \(a_{20} = 46\), 求 \(s = a_1 + a_2 + \dots + a_{20} + T_1 (P) + T_2 (P) + \dots + T_{46} (P)\) 的值.</li>
</ol>


                        
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                  <h1>不动点为复数值的递推数列问题</h1>
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                        	<p>正数数列 \(\{a_n\}\) 满足 \(S_n + 1 = \dfrac{S_n+4}{2a_n - S_n}\), 其中 \(S_n\) 是 \(\{a_n\}\) 的前 \(n\) 项和, 求 \(\{a_n\}\) 的通项公式.</p>


                        
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                  <h1>三角不等式/极值问题</h1>
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                        	<p><mark>TODO: 本题确认伪证了</mark></p>

<p>已知 \(A,B,C\) 为三角形三内角. 求 \(\dfrac{\cos^2 A}{1+\cos A} + \dfrac{\cos^2 B}{1+\cos B} + \dfrac{\cos^2 C}{1+\cos C}\) 的最小值.</p>


                        
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                  <h1>一组高中联赛二试模拟/选拔题 `zhangboxin|20180516`</h1>
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                        	<p><mark>TODO: 微博图床已挂, 本篇缺图, 需要找回</mark></p>

<p><mark>TODO: 本篇内容也还没完成</mark></p>

<p>这是二试难度的问题 (rdfz 20180516 高联选拔题)</p>


                        
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                  <h1>Poncelet 定理: 圆和抛物线 `zhangboxin|20180409`</h1>
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                        	<p>已知圆 \(C_1 \colon x^2+y^2 = 1\) 和抛物线 \(C_2 \colon y = x^2 - 2\). \(P,Q,R\) 是抛物线 \(C_2\) 上的三个不同的点, 且直线 \(PQ\) 和 \(PR\) 都是圆 \(C_1\) 的切线. 求证: \(QR\) 也是圆 \(C_1\) 的切线.</p>


                        
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                  <h1>平面几何 `duyan|20180123|2`</h1>
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                        	<p><mark>TODO: 微博图床已挂, 本篇缺图, 需要找回</mark></p>

<p>已知 \(H\) 为 \(\triangle ABC\) 的垂心, 过 \(H\) 的直线交 \(BC, AB\) 于 \(D,Z\), 过 \(H\) 且垂直于 \(ZH\) 的另一条直线交 \(BC, AC\) 于 \(E,X\), 点 \(Y\) 使得 \(DY \parallel AC, EY \parallel AB\). 求证: \(X,Y,Z\) 三点共线.</p>

<p><img src="https://ws2.sinaimg.cn/large/006tKfTcly1fnqjnu4ou9j30iu0h7js7.jpg" alt=""/></p>


                        
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                  <h1>USAMO19xx #5, 高斯函数, 不等式, 第二数学归纳法</h1>
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                        	<p>证明不等式: <br/>
\[<br/>
[nx] \ge \dfrac{[x]}1 + \dfrac{[2x]}2 + \dfrac{[3x]}3 + \dots +\dfrac{[nx]}n<br/>
\]<br/>
其中, \(n \in \mathbb N^*\), \([x]\) 表示不大于 \(x\) 的最大整数.</p>


                        
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                  <h1>一道难度其实不大的平面几何长度计算类证明题 `zhangboxin|20171218`</h1>
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                        	<p><mark>TODO: 微博图床已挂, 本篇缺图, 需要找回</mark></p>

<p>在 \(\triangle ABC\) 中, \(\angle A = 4 \angle C\), \(\angle B = 2 \angle C\), 试证: \((BC+CA) \cdot AB = BC \cdot CA\).</p>


                        
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                  <h1>一个常规的递推数列不动点法问题及其引申 `zhangzhiyuan|20171125`</h1>
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                        	<p><mark>TODO: 本文暴露出了我曾经的数学基础的弱点, 需要修改</mark></p>

<p>数列 \(\{a_n\}\) 满足 \(a_1 = 2\), \(a_{n+1} = \dfrac{a_n}2 + \dfrac1{a_n}\), 求其通项公式.</p>


                        
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                  <h1>lixinyi 20171119</h1>
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                        	<p>已知函数 \(f(x) = \sqrt{x^2+1}-ax\) (\(a \in \mathbb R\)).</p>

<p>(1) 当 \(a=1\) 时, 判断 \(f(x)\) 在 \(\mathbb R\) 上的点调性;<br/>
(2) 求实数 \(a\) 的取值范围, 使得函数 \(f(x)\) 在 \(\mathbb R^+\) 上是单调函数.</p>


                        
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                  <h1>libinglun 20171112</h1>
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                        	<p>设 \(p\) 和 \(q\) 是两个质数, 求不能表示为 \(mp + nq\) (其中, \(m,n\) 是自然数) 的最大正整数 (用 \(p\) 和 \(q\) 表示).</p>


                        
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                  <h1>组合极值/组合构造问题 `kongyuqing|20171024`</h1>
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                        	<p>100 个数围成一圈, \(a_{3} &gt; a_{2} + a_{1}\), \(a_{4} &gt; a_{3} + a_{2}\), \(\dots\) , \(a_{100} &gt; a_{99} + a_{98}\), \(a_{1} &gt; a_{100} + a_{99}\), \(a_{2} &gt; a_{1} + a_{100}\), 问其中最多有多少个正数?</p>


                        
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                  <h1>递推数列推出组合数的问题 `duyan@bj#4|20171012`</h1>
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                        	<p>已知 \(S_n\) 为数列 \(\{a_n\}\) 的前 \(n\) 项和, 规定 \(S_0 = 0\), 若对任意 \(n \in \mathbb N^*\), 均有 \(\dfrac{a_n}{2017} = - \dfrac{2017+S_{n-1}}{n}\), 则 \(\sum\limits_{n=1}^{2017} 2^n a_n = \) <code>______</code>.</p>


                        
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                  <h1>三圆轮换对称不等式问题 `mawentao|20170914`</h1>
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                        	<p><code>问题</code> \(a,b,c \ge 0\), 求证:<br/>
\[<br/>
\begin{aligned}<br/>
&amp;\sqrt{a^2 - ab + b^2} \cdot \sqrt{b^2 - bc + c^2} \\<br/>
&amp;{}+ \sqrt{b^2 - bc + c^2} \cdot \sqrt{c^2 - ca + a^2} \\<br/>
&amp;{}+ \sqrt{c^2 - ca + a^2} \cdot \sqrt{a^2 - ab + b^2}<br/>
\ge a^2 + b^2 + c^2<br/>
\end{aligned}<br/>
\]</p>


                        
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