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Algebra Difficulty 3.6 AMC 10/12 Prove it North Macedonia
Prove that1 1 ⋅ 2013 + 1 2 ⋅ 2012 + 1 3 ⋅ 2011 + ⋯ + 1 2012 ⋅ 2 + 1 2013 ⋅ 1 < 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.
1 ⋅ 2013 1 + 2 ⋅ 2012 1 + 3 ⋅ 2011 1 + ⋯ + 2012 ⋅ 2 1 + 2013 ⋅ 1 1 < 1.
Докажи дека1 1 ⋅ 2013 + 1 2 ⋅ 2012 + 1 3 ⋅ 2011 + ⋯ + 1 2012 ⋅ 2 + 1 2013 ⋅ 1 < 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.
1 ⋅ 2013 1 + 2 ⋅ 2012 1 + 3 ⋅ 2011 1 + ⋯ + 2012 ⋅ 2 1 + 2013 ⋅ 1 1 < 1.
Solutions — 2 Solution 1 For arbitrary natural numbers n ≠ 1 ≠ m n \neq 1 \neq m n = 1 = m the inequality n m ≥ n + m nm \geq n+m nm ≥ n + m holds, since ( n − 1 ) ( m − 1 ) ≥ 1 ⇒ n m − n − m + 1 ≥ 1 ⇒ n m − n − m ≥ 0 ⇒ n m ≥ n + 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 ( n − 1 ) ( m − 1 ) ≥ 1 ⇒ nm − n − m + 1 ≥ 1 ⇒ nm − n − m ≥ 0 ⇒ nm ≥ n + m , with equality only when n = m = 2 n=m=2 n = m = 2 . Then for n ≥ 2 n \geq 2 n ≥ 2 , we have1 n ( 2014 − n ) < 1 n + 2014 − n = 1 2014
\frac{1}{n(2014-n)} < \frac{1}{n+2014-n} = \frac{1}{2014}
n ( 2014 − n ) 1 < n + 2014 − n 1 = 2014 1 and hence:1 1 ⋅ 2013 + 1 2 ⋅ 2012 + 1 3 ⋅ 2011 + ⋯ + 1 2012 ⋅ 2 + 1 2013 ⋅ 1 < 2 2013 + 2011 2014 < 3 2014 + 2011 2014 = 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.
1 ⋅ 2013 1 + 2 ⋅ 2012 1 + 3 ⋅ 2011 1 + ⋯ + 2012 ⋅ 2 1 + 2013 ⋅ 1 1 < 2013 2 + 2014 2011 < 2014 3 + 2014 2011 = 1.
Solution 2 1 1 ⋅ 2013 + 1 2 ⋅ 2012 + 1 3 ⋅ 2011 + ⋯ + 1 2012 ⋅ 2 + 1 2013 ⋅ 1 = = 1 2014 ( 1 + 2013 1 ⋅ 2013 + 2 + 2012 2 ⋅ 2012 + 3 + 2011 3 ⋅ 2011 + ⋯ + 2012 + 2 2012 ⋅ 2 + 2013 + 1 2013 ⋅ 1 ) = = 1 2014 ( ( 1 1 + 1 2013 ) + ( 1 2 + 1 2012 ) + ⋯ + ( 1 2012 + 1 2 ) + ( 1 2013 + 1 1 ) ) = = 1 2014 ( 2 ( 1 1 + 1 2 + ⋯ + 1 2012 ) + 1 2013 ) = 1 2014 ( 3 + 2 3 + 2 4 + ⋯ + 2 2012 + 2 2013 ) < < 1 2014 ( 3 + 1 + 1 + ⋯ + 1 + 1 2014 ) = 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}
1 ⋅ 2013 1 + 2 ⋅ 2012 1 + 3 ⋅ 2011 1 + ⋯ + 2012 ⋅ 2 1 + 2013 ⋅ 1 1 = = 2014 1 ( 1 ⋅ 2013 1 + 2013 + 2 ⋅ 2012 2 + 2012 + 3 ⋅ 2011 3 + 2011 + ⋯ + 2012 ⋅ 2 2012 + 2 + 2013 ⋅ 1 2013 + 1 ) = = 2014 1 ( ( 1 1 + 2013 1 ) + ( 2 1 + 2012 1 ) + ⋯ + ( 2012 1 + 2 1 ) + ( 2013 1 + 1 1 ) ) = = 2014 1 ( 2 ( 1 1 + 2 1 + ⋯ + 2012 1 ) + 2013 1 ) = 2014 1 ( 3 + 3 2 + 4 2 + ⋯ + 2012 2 + 2013 2 ) < < 2014 1 ( 3 + 2014 1 + 1 + ⋯ + 1 + 1 ) = 1
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