Geometric Progression

Last Updated : 12 Dec, 2018

A sequence of numbers is called a Geometric progression if the ratio of any two consecutive terms is always same. In simple terms, it means that next number in the series is calculated by multiplying a fixed number to the previous number in the series.For example, 2, 4, 8, 16 is a GP because ratio of any two consecutive terms in the series (common difference) is same (4 / 2 = 8 / 4 = 16 / 8 = 2).

Fact about Geometric Progression :

Initial term: In a geometric progression, the first number is called the initial term.
Common ratio: The ratio between a term in the sequence and the term before it is called the “common ratio.”
The behaviour of a geometric sequence depends on the value of the common ratio. If the common ratio is:

Positive, the terms will all be the same sign as the initial term.
Negative, the terms will alternate between positive and negative.
Greater than 1, there will be exponential growth towards positive or negative infinity (depending on the sign of the initial term).
1, the progression is a constant sequence.
Between -1 and 1 but not zero, there will be exponential decay towards zero.
-1, the progression is an alternating sequence.
Less than -1, for the absolute values there is exponential growth towards (unsigned) infinity, due to the alternating sign.

Formula of n^th term of an Geometric Progression :
If ‘a’ is the first term and ‘r’ is the common ratio.Thus, the explicit formula is

Formula of sum of n^th term of Geometric Progression:

How we check whether a series is geometric progression or not?

Let’s find the ratio of the consecutive terms, so we can say that the ratio of the consecutive terms of given sequences is 13 or a constant. So this sequence is forming a geometrical progression.

C++

// C++ program to check if a given array 
// can form geometric progression 
#include <bits/stdc++.h> 
using namespace std; 
  
bool is_geometric(int arr[], int n) 
{ 
    if (n == 1) 
        return true; 
  
    // Calculate ratio 
    int ratio = arr[1] / (arr[0]); 
  
    // Check the ratio of the remaining 
    for (int i = 1; i < n; i++) { 
        if ((arr[i] / (arr[i - 1])) != ratio) { 
            return false; 
        } 
    } 
    return true; 
} 
  
// Driven Program 
int main() 
{ 
    int arr[] = { 2, 6, 18, 54 }; 
    int n = sizeof(arr) / sizeof(arr[0]); 
  
    (is_geometric(arr, n)) ? (cout << "True" << endl) : (cout << "False" << endl); 
  
    return 0; 
} 

Java

// Java program to check if a given array 
// can form geometric progression 
import java.util.Arrays; 
  
class GFG { 
  
    // function to check series is 
    // geometric progression or not 
    static boolean is_geometric(int arr[], int n) 
    { 
        if (n == 1) 
            return true; 
  
        // Calculate ratio 
        int ratio = arr[1] / (arr[0]); 
  
        // Check the ratio of the remaining 
        for (int i = 1; i < n; i++) { 
            if ((arr[i] / (arr[i - 1])) != ratio) { 
                return false; 
            } 
        } 
        return true; 
    } 
  
    // driver code 
    public static void main(String[] args) 
    { 
        int arr[] = { 2, 6, 18, 54 }; 
        int n = arr.length; 
  
        if (is_geometric(arr, n)) 
            System.out.println("True"); 
        else
            System.out.println("False"); 
    } 
} 

Python3

def is_geometric(li): 
    if len(li) <= 1: 
        return True
          
    # Calculate ratio 
    ratio = li[1]/float(li[0]) 
      
    # Check the ratio of the remaining 
    for i in range(1, len(li)): 
        if li[i]/float(li[i-1]) != ratio:  
            return False
    return True
  
print(is_geometric([2, 6, 18, 54])) 

C#

// C# program to check if a given array 
// can form geometric progression 
using System; 
  
class Geeks { 
  
    static bool is_geometric(int[] arr, int n) 
    { 
        if (n == 1) 
            return true; 
  
        // Calculate ratio 
        int ratio = arr[1] / (arr[0]); 
  
        // Check the ratio of the remaining 
        for (int i = 1; i < n; i++) { 
            if ((arr[i] / (arr[i - 1])) != ratio) { 
                return false; 
            } 
        } 
        return true; 
    } 
  
    // Driven Program 
    public static void Main(String[] args) 
    { 
        int[] arr = new int[] { 2, 6, 18, 54 }; 
        int n = arr.Length; 
  
        if (is_geometric(arr, n)) 
            Console.WriteLine("True"); 
        else
            Console.WriteLine("False"); 
    } 
} 

PHP

<?php 
function is_geometric($arr) 
{ 
    if (sizeof($arr) <= 1) 
        return True; 
    # Calculate ratio 
    $ratio = $arr[1]/$arr[0]; 
     
    # Check the ratio of the remaining 
    for($i=1; $i<sizeof($arr); $i++) 
    { 
        if (($arr[$i]/($arr[$i-1])) != $ratio) 
        { 
            return "Not a geometric sequence"; 
        } 
    }         
  return "Geometric  sequence"; 
} 
$my_arr1 = array(2, 6, 18, 54); 
  
  
print_r(is_geometric($my_arr1)."\n"); 
print_r(is_geometric($my_arr2)."\n"); 
?> 

Output:

True

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