Difference between revisions of "2019 AMC 10A Problems/Problem 15"

(Created page with "{{duplicate|2019 AMC 10A #15 and 2019 AMC 12A #9}} ==Problem== A sequence of numbers is defined recursively by <math>a_1...")
 
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==Solution==
 
==Solution==
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Using the recursive formula, we find <math>a_3=\frac{3}{10}</math>, <math>a_4=\frac{3}{14}</math>, and so on. It appears that <math>a_n=\frac{3}{4n-1}</math>, for all <math>n</math>. Setting <math>n=2019</math>, we find <math>a_{2019}=\frac{3}{8075}</math>, so the answer is <math>\boxed{\textbf{(E) }8078}</math>.
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To prove this formula, we use induction. We are given that <math>a_1=1</math> and <math>a_2=\frac{3}{7}</math>, which satisfy our formula. Now assume the formula holds true for all <math>n\le m</math> for some positive integer <math>m</math>. By our assumption, <math>a_{m-1}=\frac{3}{4m-5}</math> and <math>a_m=\frac{3}{4m-2}</math>. Using the recursive formula, <cmath>a_{m+1}=\frac{a_{m-1}\cdot a_m}{2a_{m-1}-a_m}=\frac{\frac{3}{4m-5}\cdot\frac{3}{4m-2}}{2\cdot\frac{3}{4m-5}-\frac{3}{4m-2}}=\frac{(\frac{3}{4m-5}\cdot\frac{3}{4m-2})(4m-5)(4m-2)}{(2\cdot\frac{3}{4m-5}-\frac{3}{4m-2})(4m-5)(4m-2)}=\frac{9}{6(4m-2)-3(4m-5)}=\frac{3}{4m+1},</cmath>
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so our induction is complete.
  
 
==See Also==
 
==See Also==

Revision as of 16:26, 9 February 2019

The following problem is from both the 2019 AMC 10A #15 and 2019 AMC 12A #9, so both problems redirect to this page.

Problem

A sequence of numbers is defined recursively by $a_1 = 1$, $a_2 = \frac{3}{7}$, and \[a_n=\frac{a_{n-2} \cdot a_{n-1}}{2a_{n-2} - a_{n-1}}\]for all $n \geq 3$ Then $a_{2019}$ can be written as $\frac{p}{q}$, where $p$ and $q$ are relatively prime positive inegers. What is $p+q ?$

$\textbf{(A) } 2020 \qquad\textbf{(B) } 4039 \qquad\textbf{(C) } 6057 \qquad\textbf{(D) } 6061 \qquad\textbf{(E) } 8078$

Solution

Using the recursive formula, we find $a_3=\frac{3}{10}$, $a_4=\frac{3}{14}$, and so on. It appears that $a_n=\frac{3}{4n-1}$, for all $n$. Setting $n=2019$, we find $a_{2019}=\frac{3}{8075}$, so the answer is $\boxed{\textbf{(E) }8078}$.


To prove this formula, we use induction. We are given that $a_1=1$ and $a_2=\frac{3}{7}$, which satisfy our formula. Now assume the formula holds true for all $n\le m$ for some positive integer $m$. By our assumption, $a_{m-1}=\frac{3}{4m-5}$ and $a_m=\frac{3}{4m-2}$. Using the recursive formula, \[a_{m+1}=\frac{a_{m-1}\cdot a_m}{2a_{m-1}-a_m}=\frac{\frac{3}{4m-5}\cdot\frac{3}{4m-2}}{2\cdot\frac{3}{4m-5}-\frac{3}{4m-2}}=\frac{(\frac{3}{4m-5}\cdot\frac{3}{4m-2})(4m-5)(4m-2)}{(2\cdot\frac{3}{4m-5}-\frac{3}{4m-2})(4m-5)(4m-2)}=\frac{9}{6(4m-2)-3(4m-5)}=\frac{3}{4m+1},\] so our induction is complete.

See Also

2019 AMC 10A (ProblemsAnswer KeyResources)
Preceded by
Problem 14
Followed by
Problem 16
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
All AMC 10 Problems and Solutions
2019 AMC 12A (ProblemsAnswer KeyResources)
Preceded by
Problem 8
Followed by
Problem 10
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
All AMC 12 Problems and Solutions

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