Difference between revisions of "2022 AIME II Problems/Problem 5"
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− | Note: I believe that you also need <math>20 \geq a</math>, so you cannot just simply do <math>18 \cdot 4</math>. In addition, it is possible for <cmath>b - c = 2</cmath> <cmath>a - b = p_1</cmath> to occur. You would need to do casework to make sure <math>a</math> does not go above 20. | + | Note: I believe that you also need <math>20 \geq a</math>, so you cannot just simply do <math>18 \cdot 4</math>. In addition, it is possible for <cmath>b - c = 2</cmath> <cmath>a - b = p_1</cmath> to occur. You would need to do casework to make sure <math>a</math> does not go above 20. |
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+ | If you did the casework, it should come out to be <math>(15+13+7+1) \cdot 2 = \boxed{\textbf{072}}</math>. It just so happens that <math>72 = 18 \cdot 4</math>. | ||
==See Also== | ==See Also== | ||
{{AIME box|year=2022|n=II|num-b=4|num-a=6}} | {{AIME box|year=2022|n=II|num-b=4|num-a=6}} | ||
{{MAA Notice}} | {{MAA Notice}} |
Revision as of 12:46, 30 August 2022
Problem
Twenty distinct points are marked on a circle and labeled through in clockwise order. A line segment is drawn between every pair of points whose labels differ by a prime number. Find the number of triangles formed whose vertices are among the original points.
Solution
Let , , and be the vertex of a triangle that satisfies this problem, where .
. Because is the sum of two primes, and , or must be . Let , then . There are only primes less than : . Only plus equals another prime. .
Once is determined, and . There are values of where , and values of . Therefore the answer is
Note: I believe that you also need , so you cannot just simply do . In addition, it is possible for to occur. You would need to do casework to make sure does not go above 20.
If you did the casework, it should come out to be . It just so happens that .
See Also
2022 AIME II (Problems • Answer Key • Resources) | ||
Preceded by Problem 4 |
Followed by Problem 6 | |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 | ||
All AIME Problems and Solutions |
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