Difference between revisions of "2023 AMC 12A Problems/Problem 20"

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(no solutions here yet!)
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==Problem==
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Rows 1, 2, 3, 4, and 5 of a triangular array of integers are shown below.
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==Solution 1==
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First, let <math>R(n) be the sum of the </math>n<math>th row. Now, with some observations and math instinct, we can guess that </math>R(n) = 2^n - n<math>
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now we try to prove it by induction,
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</math>R(1) = 2^n - n = 2^1 - 1 = 1<math> (works for base case!)
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</math>R(k) = 2^k - k<math>
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</math>R(k+1) = 2^{k+1} - (k + 1) = 2(2^k) - k - 1<math>
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Now by definition from the question, the next row is always: double the sum of last row (Each number from last row branches off toward left and right to the next row), plus # of new row, minus 2 (leftmost and rightmost are just 1)
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</math>2(2^k - k) + (k + 1) - 2 = 2(2^k) - k - 1<math>
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Hence proven.
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Simply substitute </math>n = 2023<math>, we get </math>R(2023) = 2^2023 - 2023$
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Last digit of 2^2023 is 8, 8-3 = \boxed{\textbf{(C)} 5}
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~lptoggled
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==See also==
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{{AMC12 box|year=2023|ab=A|num-b=19|num-a=21}}
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{{MAA Notice}}

Revision as of 21:42, 9 November 2023

Problem

Rows 1, 2, 3, 4, and 5 of a triangular array of integers are shown below.

Solution 1

First, let $R(n) be the sum of the$n$th row. Now, with some observations and math instinct, we can guess that$R(n) = 2^n - n$now we try to prove it by induction,$R(1) = 2^n - n = 2^1 - 1 = 1$(works for base case!)$R(k) = 2^k - k$$ (Error compiling LaTeX. Unknown error_msg)R(k+1) = 2^{k+1} - (k + 1) = 2(2^k) - k - 1$Now by definition from the question, the next row is always: double the sum of last row (Each number from last row branches off toward left and right to the next row), plus # of new row, minus 2 (leftmost and rightmost are just 1)$ (Error compiling LaTeX. Unknown error_msg)2(2^k - k) + (k + 1) - 2 = 2(2^k) - k - 1$Hence proven.

Simply substitute$ (Error compiling LaTeX. Unknown error_msg)n = 2023$, we get$R(2023) = 2^2023 - 2023$

Last digit of 2^2023 is 8, 8-3 = \boxed{\textbf{(C)} 5}

~lptoggled

See also

2023 AMC 12A (ProblemsAnswer KeyResources)
Preceded by
Problem 19
Followed by
Problem 21
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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