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Algebra Difficulty 6.2 National olympiad Prove it

4.

(a) Show that for any real numbers a,b,ca, b, c we have a+b+a+cbc|a+b|+|a+c| \geqslant |b-c|.

(b) Prove that for any real number xx we have

x+1+x+2+x+3++x+201410072 |x+1|+|x+2|+|x+3|+\ldots+|x+2014| \geqslant 1007^{2}

Solution

Solution.

(a) Knowing that x+yxya+b+a+ca+bac=bc|x|+|y| \geqslant|x-y| \Longrightarrow|a+b|+|a+c| \geqslant|a+b-a-c|=|b-c|. (2p)

(b) Applying, in turn, the relation shown in (a), we obtain

x+1+x+2014x+2014x1=2013x+2+x+2013x+2013x2=2011++1008x1007=1 \begin{aligned} |x+1|+|x+2014| & \geqslant|x+2014-x-1|=2013 \\ |x+2|+|x+2013| & \geqslant|x+2013-x-2|=2011 \\ \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots & \cdots \cdots+\ldots+1008-x-1007 \mid=1 \end{aligned}

Summing the inequalities, we obtain:

x+1+x+2++x+20141+3++2011+2013=10072 |x+1|+|x+2|+\ldots+|x+2014| \geqslant 1+3+\ldots+2011+2013=1007^{2}

where

1+3++2011+2013=(1+2++2014)(2+4++2014)=2014201522100710082=10072 \begin{aligned} 1+3+\ldots+2011+2013 & =(1+2+\ldots+2014)-(2+4+\ldots+2014) \\ & =\frac{2014 \cdot 2015}{2}-2 \frac{1007 \cdot 1008}{2}=1007^{2} \end{aligned}

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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.