23 lines
469 B
C
23 lines
469 B
C
#include <stdio.h>
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#include <stdlib.h>
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#include <math.h>
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#include <stdbool.h>
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bool isPentagonal (long long n) {
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long long radicand = 24*n + 1;
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long long sqrtr = sqrt (radicand);
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return sqrtr * sqrtr == radicand && (sqrtr + 1) % 6 == 0;
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}
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int main () {
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long long n = 144LL;
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while (true) {
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long long hex = n * (2*n - 1);
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if (isPentagonal (hex)) {
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printf ("%lli\n", hex);
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break;
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}
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n++;
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}
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}
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