程序以找到系列3，5，21，51，95，…的第N个术语

给定数字N，任务是找到该系列的第N个项：

3, 5, 21, 51, 95, ….

例子：

Input: N = 4
Output: 51
Explanation:
Nth term = (7 * pow(N, 2) - 19 * N + 15)
         = (7 * pow(4, 2) - 19 * 4 + 15)
         = 51

Input: N = 10
Output: 525

方法：
给定级数的N个项可以概括为：

$T_n = 7 * n * n - 14 * n + 15$ [Tex] [/ Tex]

下面是上述方法的实现：

C++

// CPP program to find N-th term of the series:
// 3, 5, 21, 51, 95,...
 
#include 
#include 
using namespace std;
 
// calculate Nth term of series
int getNthTerm(long long int N)
{
    // Return Nth term
    return (7 * pow(N, 2) - 19 * N + 15);
}
 
// driver code
int main()
{
    // declaration of number of terms
    long long int N = 4;
 
    // Get the Nth term
    cout << getNthTerm(N);
 
    return 0;
}

Java

// Java program to find N-th term of the series:
// 3, 5, 21, 51, 95,...
import java.util.*;
 
class solution
{
static long getNthTerm(long N)
{
// Return Nth term
    return (7 *(int) Math.pow(N, 2) - 19 * N + 15);
}
 
//Driver program
public static void main(String arr[])
{
// declaration of number of terms
    long N = 4;
 
    // Get the Nth term
     System.out.println(getNthTerm(N));
}
}

Python3

# Python3 program to find N-th term
# of the series:
# 3, 5, 21, 51, 95,...
 
# calculate Nth term of series
def getNthTerm(N):
     
    #Return Nth term
    return (7 * pow(N, 2) - 19 * N + 15)
     
#Driver code
if __name__=='__main__':
     
#declaration of number of terms
    N = 4
 
#Get the Nth term
    print(getNthTerm(N))
 
#this code is contributed by Shashank_Sharma

C#

// C# program to find
// N-th term of the series:
// 3, 5, 21, 51, 95,...
using System;
 
class GFG
{
static long getNthTerm(long N)
{
    // Return Nth term
    return (7 *(int) Math.Pow(N, 2) -
                       19 * N + 15);
}
 
// Driver Code
static public void Main ()
{
 
    // declaration of number
    // of terms
    long N = 4;
 
    // Get the Nth term
    Console.Write(getNthTerm(N));
}
}
 
// This code is contributed by Raj

Javascript

 
// JavaScript  program to find N-th term of the series:
// 3, 5, 21, 51, 95,...
 
 
// calculate Nth term of series
function getNthTerm(N)
{
    // Return Nth term
    return (7 * Math.pow(N, 2) - 19 * N + 15);
}
 
// driver code
  // declaration of number of terms
   let N = 4;
   document.write( getNthTerm(N) );
 
// This code contributed by gauravrajput1
 

输出：

51

时间复杂度： O(1)