A C program code for finding the summation of the series (a+1)+(a+2)+.....(a+n) using a for loop.
```c
#include <stdio.h>
int main() {
int a, n, sum = 0;
printf("Enter the value of a: ");
scanf("%d", &a);
printf("Enter the value of n: ");
scanf("%d", &n);
for (int i = 1; i <= n; i++) {
sum += (a + i);
}
printf("Sum of the series is: %d", sum);
return 0;
}
```