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# 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, 𝑎𝑟^{2} , 𝑎𝑟^{3} , 𝑎𝑟^{4} , … and their sum, i.e., a + ar + 𝑎𝑟^{2}+ 𝑎𝑟^{3} + 𝑎𝑟^{4} +…

**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

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