2+5+8+11+……….up to n’th term.

#include<stdio.h>
#include<conio.h>

void main()
{
    int n,i,s=0;
    clrscr();
    printf("Enter how many number you sum = ");
    scanf("%d",&n);
   
    for(i=1;i<=n;i++)
    s=s+3*i-1;

    printf("Summation = %d",s);
   
    getch();
}

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Copy & paste this code in your TC & run, then you will get output.......
If you have any problem please comment below.........