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Algebra Difficulty 7.1 National olympiad, round 2 Find the answer

Example 6 Real numbers x1,x2,,x2001x_{1}, x_{2}, \cdots, x_{2001} satisfy k=52000xkxk+1=200 F\sum_{k=5}^{2000}\left|x_{k}-x_{k+1}\right|=200 \mathrm{~F}, let yk=1k(x1+y_{k}=\frac{1}{k}\left(x_{1}+\right. x2++xk),k=1,2,2001\left.x_{2} + \cdots+x_{k}\right), k=1,2-\cdots, 2001. Find the maximum possible value of k=12000ykyk+1\sum_{k=1}^{2000} \left| y_{k}-y_{k+1} \right|. (2001 Upper

A number or a short expression. Spacing and $ signs are ignored.

Solution

For k=1,2,,2000k=1,2, \cdots, 2000, we have
ykyk+1=1k(x1+x2++xk)1k+1(x1+x2++xk+1)=x1+x2++xkkxk+1k(k+1)=(x1x2)+2(x2x3)++k(xkxk+1)k(k+1)x1x2+2x2x3++kxkxk+1k(k+1)\begin{array}{l} \left|y_{k}-y_{k+1}\right|=\left|\frac{1}{k}\left(x_{1}+x_{2}+\cdots+x_{k}\right)-\frac{1}{k+1}\left(x_{1}+x_{2}+\cdots+x_{k+1}\right)\right|= \\ \left|\frac{x_{1}+x_{2}+\cdots+x_{k}-k x_{k+1}}{k(k+1)}\right|= \\ \frac{\left|\left(x_{1}-x_{2}\right)+2\left(x_{2}-x_{3}\right)+\cdots+k\left(x_{k}-x_{k+1}\right)\right|}{k(k+1)} \leqslant \\ \frac{\left|x_{1}-x_{2}\right|+2\left|x_{2}-x_{3}\right|+\cdots+k\left|x_{k}-x_{k+1}\right|}{k(k+1)} \end{array}

By the identity
11×2+12×3++1(n1)n=(112)+(1213)++(1n11n)=11n\begin{aligned} \frac{1}{1 \times 2}+\frac{1}{2 \times 3}+\cdots+\frac{1}{(n-1) n}= & \left(1-\frac{1}{2}\right)+\left(\frac{1}{2}-\frac{1}{3}\right)+\cdots+\left(\frac{1}{n-1}-\frac{1}{n}\right)= \\ & 1-\frac{1}{n} \end{aligned}

and its result
1k(k+1)+F(k+1)(k+2)++Γ(n1)n=Fk(1kn)\frac{1}{k(k+1)}+\frac{F}{(k+1)(k+2)}+\cdots+\frac{\Gamma}{(n-1) \cdot n}=\frac{F}{k}\left(1-\frac{k}{n}\right)

we get
k=F2000=Lykyk+1x1x2(11×2+12×3++12000×2001)+2x2x3(12×3+134++12000×2001)+++2000x2000x200112000×2001=x1x2(112001)+x2x3(122001)++x2000x200F(120002001)x1x2(112001)+x2x3(112001)++x2000x2000(112001)=\begin{aligned} \sum_{k=F}^{2000=} L y_{k}-y_{k+1} \mid \leqslant & \left|x_{1}-x_{2}\right|\left(\frac{1}{1 \times 2}+\frac{1}{2 \times 3}+\cdots+\frac{1}{2000 \times 2001}\right)+ \\ & 2\left|x_{2}-x_{3}\right|\left(\frac{1}{2 \times 3}+\frac{1}{3-4}+\cdots+\frac{1}{2000 \times 2001}\right)+\cdots++ \\ & 2000 \cdot\left|x_{2000}-x_{2001}\right| \cdot \frac{1}{2000 \times 2001}= \\ & \left|x_{1}-x_{2}\right|\left(1-\frac{1}{2001}\right)+\left|-x_{2}-x_{3}\right|\left(1-\frac{2}{2001}\right)+\cdots+ \\ & \left|x_{2000}-x_{200 F}\right|\left(1-\frac{2000}{2001}\right) \leqslant \\ & \left|x_{1}-x_{2}\right|\left(1-\frac{1}{2001}\right)+\left|x_{2}-x_{3}\right|\left(1-\frac{1}{2001}\right)+\cdots+\cdots \\ & \left|x_{2000}-x_{2000}\right|-\left(1-\frac{1}{2001}\right)= \end{aligned}

Equality holds if and only if xFx2=2001,x2=x3==x2001\left|x_{F}-x_{2}\right|=2001, x_{2}=x_{3}=\cdots=x_{2001}, specifically by taking x1=2001x_{1}=2001, x2=x3==x200=0x_{2}=x_{3}=\cdots=x_{200 ₹}=0 the equality can be achieved.

Therefore, the maximum value of k=12000yk=yk+1\sum_{k=1}^{2000}-y_{k}=y_{k+1} is 2000.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.