# WAP in C++ to display sum of series [ 9 + 99 + 999 + 9999 …]

The program in C++ to display sum of series [ 9 + 99 + 999 + 9999 …] is given below:

``````#include <iostream>
using namespace std;

int main() {
int n;
long long sum = 0;

cout << "Hello Codeauri Family,enter the number of terms in the series here: \n";
cin >>n;

for (int i = 1; i <= n; i++) {
int num = 0;
for (int j = 1; j <= i; j++) {
num = num * 10 + 9;
}
sum += num;
}
cout << "The sum of the series [9 + 99 + 999 + 9999 ...] for " << n << " terms is: " << sum << endl;

return 0;
}
``````

## Output:

Hello Codeauri Family,enter the number of terms in the series here:
10
The sum of the series [9 + 99 + 999 + 9999 …] for 10 terms is: 2521176508

## Pro-Tips💡

This program prompts the user to enter an integer value for `n`,

and then uses two nested for loops to generate the first `n` terms of the series [ 9 + 99 + 999 + 9999 …] and calculate their sum.

The sum is then displayed at the end. Since the series sum is of order n^2, it’s a good practice to use long long variables.

Note that this program uses standard input/output library <iostream>

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