tnx for the help i completed my exercise^_^. I'll start my next one later....tnx again.

Code:

#include <iostream>
#include <math.h>
using namespace std;
int main(void)
{
float num, avg = 0, avgsqr = 0, tl1, tl2, SD;
int n;
cout << "How many numbers would u like to input^_^\n";
cin >> n;
for(int a =1; a<=n; a++){
cout << "input number\n";
cin >> num;
cin.ignore();
avg += num;
num *= num;
avgsqr += num;
tl1 = avg/n;
tl2 = avgsqr/n;
SD = tl2 - tl1;
}
cout <<"avg = " << avg << "= " << tl1 << "\n";
cout << "avg2 = " << sqrt(tl1) << "\n";
cout << "avgSquare = " << avgsqr << "\n";
cout << "avgSquare/" << n << " = " << tl2 << "\n";
cout << "SD = " << sqrt(SD) << "\n";
cin.get();
}