Why am I getting a linker error? (Function template)

Question

I am getting LNK1120 error, and I am not sure why. I am trying to use a template for int and double, that returns the corresponding type.

This is my Numerical_Analysis_Functions.cpp file

#include "Numerical_Analysis_Functions.h"
#include <iostream>
using namespace std;

template double 
midpointRule<double>(double arr[], int);

template <typename T> double midpointRule(double arr[], int size)
{

    return 1.0;
}

This is my Numerical_Analysis_Functions.h file

#pragma once

template <typename T>
extern T midpointRule(T arr[], int size);

This is my main

#include <iostream>
#include "Numerical_Analysis_Functions.h"
using namespace std;

int main()
{
    double arr[4] = { 1, 2, 3, 4 };
    midpointRule(arr, 4);


    cin.sync();
    cout << "End of Program.";
    cin.get();
    return 0;
}