Answer:
// A optimized school method based C++ program to check Â
// if a number is composite. Â
#include <bits/stdc++.h> Â
using namespace std; Â
bool isComposite(int n) Â
{ Â
// Corner cases Â
if (n <= 1) return false; Â
if (n <= 3) return false; Â
// This is checked so that we can skip Â
// middle five numbers in below loop Â
if (n%2 == 0 || n%3 == 0) return true; Â
for (int i=5; i*i<=n; i=i+6) Â
 if (n%i == 0 || n%(i+2) == 0) Â
 return true; Â
return false; Â
} Â
// Driver Program to test above function Â
int main() Â
{ Â
isComposite(11)? cout << " true\n": cout << " false\n"; Â
isComposite(15)? cout << " true\n": cout << " false\n"; Â
return 0; Â
}
Explanation: