Problem 45

45.c

@@ -0,0 +1,22 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <math.h>
+#include <stdbool.h>
+
+bool isPentagonal (long long n) {
+    long long radicand = 24*n + 1;
+    long long sqrtr = sqrt (radicand);
+    return sqrtr * sqrtr == radicand && (sqrtr + 1) % 6 == 0;
+}
+
+int main () {
+    long long n = 144LL;
+    while (true) {
+        long long hex = n * (2*n - 1);
+        if (isPentagonal (hex)) {
+            printf ("%lli\n", hex);
+            break;
+        }
+        n++;
+    }
+}