Program for sum of geometric series in C

Given three inputs first one is “a” which is for the first term of geometric series second is “r” which is the common ratio and “n” which are the number of series whose sum we have to find.

Geometric series is a series which have a constant ratio between its successive terms. Using the above stated inputs “a”, “r” and “n” we have to find the geometric series i.e., a, ar, 𝑎𝑟² , 𝑎𝑟³ , 𝑎𝑟⁴ , … and their sum, i.e., a + ar + 𝑎𝑟²+ 𝑎𝑟³ + 𝑎𝑟⁴ +…

Input

a = 1
r = 0.5
n = 5

Output

1.937500

Input

a = 2
r = 2.0
n = 8

Output

510.000000

Approach used below is as follows to solve the problem

• Take all the inputs a, r, n.

• Calculate the sum of geometric series, adding the full series.

Algorithm

Start
In function float sumgeometric(float a, float r, int n)
   Step 1→Declare and Initialize sum = 0
   Step 2→ Loop For i = 0 and i < n and i++
      Set sum = sum + a
      Set a = a * r
   Step 3→ Return sum
In function int main()
   Step 1→ Declare and initialize a = 1
   Step 2→ Declare and Initialize float r = 0.5
   Step 3→ Declare and initialize n = 5
   Step 4→ Print sumgeometric(a, r, n)
Stop

Example

#include <stdio.h>
// function to calculate sum of
// geometric series
float sumgeometric(float a, float r, int n){
   float sum = 0;
   for (int i = 0; i < n; i++){
      sum = sum + a;
      a = a * r;
   }
   return sum;
}
int main(){
   int a = 1; // first term
   float r = 0.5; // their common ratio
   int n = 5; // number of terms
   printf("%f", sumgeometric(a, r, n));
   return 0;
}

Output

If run the above code it will generate the following output −

1.937500