Difference between revisions of "2022 AMC 12B Problems/Problem 23"
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Revision as of 14:24, 26 November 2022
- The following problem is from both the 2022 AMC 12B #23 and 2022 AMC 10B #25, so both problems redirect to this page.
Contents
[hide]Problem
Let be a sequence of numbers, where each is either or . For each positive integer , define
Suppose for all . What is the value of the sum
Solution
First, notice that
Then since is the modular inverse of 7 in , we can perform the Euclidean algorithm to find it for .
Starting with , Now, take both sides Using Fermat's Little Theorem, Thus, Therefore,
We may repeat this same calculation with to yield Now, we notice that is basically an integer expressed in binary form with bits. This gives rise to a simple inequality, Since the maximum possible number that can be generated with bits is Looking at our calculations for and , we see that the only valid integers that satisfy that constraint are ~
Solution 2 (Base-2 Analysis)
We solve this problem with base 2. To avoid any confusion, for a base-2 number, we index the th rightmost digit as digit .
We have .
In the base-2 representation, is equivalent to
In the rest of the analysis, to lighten notation, we ease the base-2 subscription from all numbers. The equation above can be reformulated as:
\begin{table} \begin{tabular}{ccccccccc}
& & 0 & & 0 & 0 & 0 & 0 & 0 \\ & & & & & & & & 1 \\ & & & & & & & & \\ \hline & & & & & & 0 & 0 & 0\\
\end{tabular} \end{table}
Therefore, , , and for , .
Therefore,
~Steven Chen (Professor Chen Education Palace, www.professorchenedu.com)
Solution 3
As in Solution , we note that
We also know that and , this implies:
Dividing by , we can isolate the previous sums:
The maximum value of occurs when every is equal to . Even when this happens, the value of is less than . Therefore, we can construct the following inequalities:
From these two equations, we can deduce that both and are less than .
Reducing and we see that
and
The powers of repeat every
Therefore, and Substituing this back into the above equations,
and
Since and are integers less than , the only values of and are and respectively.
The requested sum is
-Benedict T (countmath1)
Video Solutions
~ ThePuzzlr
~Steven Chen (Professor Chen Education Palace, www.professorchenedu.com)
Video Solution by OmegaLearn Using Binary and Modular Arithmetic
~ pi_is_3.14
See Also
2022 AMC 10B (Problems • Answer Key • Resources) | ||
Preceded by Problem 24 |
Followed by Last problem | |
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 |
2022 AMC 12B (Problems • Answer Key • Resources) | |
Preceded by Problem 22 |
Followed by Problem 24 |
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 |
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.