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Algebra Difficulty 3.6 AMC 10/12 Prove it North Macedonia

Prove that
112013+122012+132011++120122+120131<1. \frac{1}{1 \cdot 2013} + \frac{1}{2 \cdot 2012} + \frac{1}{3 \cdot 2011} + \dots + \frac{1}{2012 \cdot 2} + \frac{1}{2013 \cdot 1} < 1.

Докажи дека
112013+122012+132011++120122+120131<1. \frac{1}{1 \cdot 2013} + \frac{1}{2 \cdot 2012} + \frac{1}{3 \cdot 2011} + \dots + \frac{1}{2012 \cdot 2} + \frac{1}{2013 \cdot 1} < 1.

Solutions — 2

Solution 1

For arbitrary natural numbers n1mn \neq 1 \neq m the inequality nmn+mnm \geq n+m holds, since (n1)(m1)1nmnm+11nmnm0nmn+m(n-1)(m-1) \geq 1 \Rightarrow nm-n-m+1 \geq 1 \Rightarrow nm-n-m \geq 0 \Rightarrow nm \geq n+m, with equality only when n=m=2n=m=2. Then for n2n \geq 2, we have
1n(2014n)<1n+2014n=12014 \frac{1}{n(2014-n)} < \frac{1}{n+2014-n} = \frac{1}{2014}
and hence:
112013+122012+132011++120122+120131<22013+20112014<32014+20112014=1. \frac{1}{1 \cdot 2013} + \frac{1}{2 \cdot 2012} + \frac{1}{3 \cdot 2011} + \dots + \frac{1}{2012 \cdot 2} + \frac{1}{2013 \cdot 1} < \frac{2}{2013} + \frac{2011}{2014} < \frac{3}{2014} + \frac{2011}{2014} = 1.

Solution 2

112013+122012+132011++120122+120131==12014(1+201312013+2+201222012+3+201132011++2012+220122+2013+120131)==12014((11+12013)+(12+12012)++(12012+12)+(12013+11))==12014(2(11+12++12012)+12013)=12014(3+23+24++22012+22013)<<12014(3+1+1++1+12014)=1 \begin{aligned} & \frac{1}{1 \cdot 2013} + \frac{1}{2 \cdot 2012} + \frac{1}{3 \cdot 2011} + \dots + \frac{1}{2012 \cdot 2} + \frac{1}{2013 \cdot 1} = \\ &= \frac{1}{2014} \left( \frac{1+2013}{1 \cdot 2013} + \frac{2+2012}{2 \cdot 2012} + \frac{3+2011}{3 \cdot 2011} + \dots + \frac{2012+2}{2012 \cdot 2} + \frac{2013+1}{2013 \cdot 1} \right) = \\ &= \frac{1}{2014} \left( \left( \frac{1}{1} + \frac{1}{2013} \right) + \left( \frac{1}{2} + \frac{1}{2012} \right) + \dots + \left( \frac{1}{2012} + \frac{1}{2} \right) + \left( \frac{1}{2013} + \frac{1}{1} \right) \right) = \\ &= \frac{1}{2014} \left( 2 \left( \frac{1}{1} + \frac{1}{2} + \dots + \frac{1}{2012} \right) + \frac{1}{2013} \right) = \frac{1}{2014} \left( 3 + \frac{2}{3} + \frac{2}{4} + \dots + \frac{2}{2012} + \frac{2}{2013} \right) < \\ &< \frac{1}{2014} \left( 3 + \frac{1+1+\dots+1+1}{2014} \right) = 1 \end{aligned}

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Source: MathNet, licensed CC-BY-4.0. Statement and solution reproduced as published; topic and difficulty added by this site.