1) Fib numbers have a closed form.
2) Every 3rd fib number starting from fib(0) = 0 is even, (i.e. 0, 1, 1, 2, 3, 5, 8 etc)
So:
int fib(int n);
void main(){
int sum=0;
int n=0;
while(fib(n) < 4000000){
sum+=fib(n);
n+=3;
}
cout<<sum<<endl;
}
int fib(int n){
double f =...