geometric.c

#include <stdio.h>

int sum (int n, int a, int r) {
	if (n == 1) {
		return a;
	}
	return a + r * sum(n - 1, a, r);
}

int main (int argc, char *argv[]) {
	printf("%d\n", sum(10, 1, 2));
	return 0;
}