# Find nth term of a given recurrence relation

• Last Updated : 08 Apr, 2021

Let an be a sequence of numbers, which is defined by the recurrence relation a1=1 and an+1/an=2n. The task is to find the value of log2(an) for a given n.
Examples:

Input: 5
Output: 10
Explanation:
log2(an) = (n * (n - 1)) / 2
= (5*(5-1))/2
= 10

Input: 100
Output: 4950

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## C++

 // C++ program to find nth term of// a given recurrence relation #include using namespace std; // function to return required valueint sum(int n){     // Get the answer    int ans = (n * (n - 1)) / 2;     // Return the answer    return ans;} // Driver programint main(){     // Get the value of n    int n = 5;     // function call to print result    cout << sum(n);     return 0;}

## Java

 // Java program to find nth term// of a given recurrence relationimport java.util.*; class solution{static int sum(int n){    // Get the answer    int ans = (n * (n - 1)) / 2;     // Return the answer    return ans;} // Driver codepublic static void main(String arr[]){     // Get the value of n    int n = 5;     // function call to print result    System.out.println(sum(n));}}//This code is contributed byte//Surendra_Gangwar

## Python3

 # Python3 program to find nth# term of a given recurrence# relation # function to return# required valuedef sum(n):     # Get the answer    ans = (n * (n - 1)) / 2;         # Return the answer    return ans # Driver Code # Get the value of nn = 5 # function call to prresultprint(int(sum(n))) # This code is contributed by Raj

## C#

 // C# program to find nth term// of a given recurrence relationusing System; class GFG{static int sum(int n){    // Get the answer    int ans = (n * (n - 1)) / 2;     // Return the answer    return ans;} // Driver codepublic static void Main(){     // Get the value of n    int n = 5;     // function call to print result    Console.WriteLine(sum(n));}} // This code is contributed byte// inder_verma

## PHP

 

## Javascript

 
Output:
10

Time Complexity: O(1)

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