求系列 3, 7, 19, 55, 163, 的第 N 项。 . .

给定一个正整数N 。任务是找到系列 3, 7, 19, 55, 163, ….. 的第 N项

例子：

Input: N = 5
Output: 163

Input: N = 1
Output: 3

方法：使用以下模式形成序列。对于任何值 N

T_N = 2 * 3^{N – 1} + 1

插图：

Input: N = 5
Output: 163
Explanation:
T_N = 2 * 3^{N – 1} + 1
     = 2 * 3^{5 – 1} + 1
     = 2 * 81 + 1
     = 162 + 1
     = 163

下面是上述方法的实现：

C++

// C++ program to implement
// the above approach
#include 
using namespace std;
 
// Function to return Nth term
// of the series
int calcNum(int N)
{
    return 2 * pow(3, N - 1) + 1;
}
 
// Driver Code
int main()
{
    int N = 5;
    cout << calcNum(N);
    return 0;
}

Java

// Java code to implement the above approach
import java.lang.*;
public class gfg
{
     
    /* Function to return the Nth term of the series */
    static int calcNum(int N)
    {
        return (int)(2*(Math.pow(3,N-1))) + 1 ;
    }
 
    // Driver Code
    public static void main(String[] args)
    {
        int N = 5;
 
        System.out.println(calcNum(N));
    }
}
 
// This code is contributed by Abhishek Thakur

Python3

# Python3 program to implement
# the above approach
 
# Function to return Nth term
# of the series
def calcNum(N):
     
    return 2 * (3 ** (N - 1)) + 1
 
# Driver Code
N = 5
 
print(calcNum(N))
 
# This code is contributed by gfgking

C#

// C# code to implement the above approach
using System;
public class gfg
{
 
  /* Function to return the Nth term of the series */
  static int calcNum(int N)
  {
    return (int)(2 * (Math.Pow(3, N - 1))) + 1;
  }
 
  // Driver Code
  public static void Main(string[] args)
  {
    int N = 5;
 
    Console.WriteLine(calcNum(N));
  }
}
 
// This code is contributed by Abhishek Thakur

Javascript

      // JavaScript code for the above approach
 
 
      // Function to return Nth term
      // of the series
      function calcNum(N) {
          return 2 * Math.pow(3, N - 1) + 1;
      }
 
      // Driver Code
 
      let N = 5;
      document.write(calcNum(N));
 
// This code is contributed by Potta Lokesh
  

输出

163

时间复杂度： O(1)

辅助空间： O(1)