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Hello, I'm just learning C++ and I'm getting an error with a program that is driving me nuts, and I'm not exactly sure how to resolve it. Can anyone help me?

AreaCalculator.obj : error LNK2019: unresolved external symbol "double __cdecl AreaOfTriangle(double,double)" (?AreaOfTriangle@@YANNN@Z) referenced in function _main
\\.psf\Home\Documents\Visual Studio 2008\Projects\AreaCalculator\Debug\AreaCalculator.exe : fatal error LNK1120: 1 unresolved externals

#include <iostream>		//for using cin and cout
#include <conio.h>		//for using getch()
#include <cmath>		//for using pow

using namespace std;

double AreaOfRectangle(/* in */ double length,   //length of rectangle
			 /* in */ double width);   //width of rectangle
//Purpose: Calculate the area of a rectangle
//Precondition:
//         length and width are assigned the length and width of the rectangle.
//		   length > 0 and width > 0
//Postcondition:
//         Returns the area of the rectangle


double AreaOfCircle(/* in */ double radius);  //radius of circle
//Purpose: Calculate the area of a circle
//Precondition:
//         radius is assigned the radius of the circle.
//		   radius > 0 
//Postcondition:
//         Returns the area of the circle.

double AreaOfTriangle (/* in */ double base, //length of triangle base
					   /* in */ double height); //height of triangle
//Purpose:	Calculate the area of a triangle
//Precondition:
//			base and height are assigned the base and height of the triangle
//			base > 0 and height > 0
//Postcondition:
//			Returns the area of the triangle.

int main()
{
	double radius;		//radius of circle
	double length;		//length of rectangle
	double width;		//width of rectangle
	double base;		//base length of triangle
	double height;		//height of triangle
	int number;		//number entered to indicate user's selection


	
	//display a heading
	cout << "*** Calculation of area ***\n\n\n";

	//Ask user to enter 1 or 2 and read the number
	cout << "Enter 1 to calculate the area of a rectangle, \n";
	cout << "      2 to calculate the area of a circle, or\n";
	cout << "      3 to calculate the area of a triangle.\n\n";
	cout << "Please enter 1, 2, or 3: ";
	cin >> number;
	cout << endl;

	//calculate area of rectangle or area of circle 
	if (number == 1)	//for calculating the area of rectangle
	{
	   //prompt for and read the length and width of rectangle
	   cout << "Enter the length of the rectangle in inches: ";  
	   cin >> length;		
	   cout << "Enter the width of the rectangle in inches: ";	
	   cin >> width;		
		
	   //Call function to calculate area of rectangle, and display the area
	   cout << endl << "Area of rectangle is: " << AreaOfRectangle(length,width)
 	        << " square inches.\n\n";					  
	}
	else if (number == 2 )	//for calculating the area of circle
	{
	     //prompt for and read the radius of circle
	     cout << "Enter the radius of the circle in inches: ";
	     cin >> radius;

	     //Call function to calculate area of circle, and display the area
	     cout << endl << "Area of circle is " << AreaOfCircle(radius) << " square inches.\n\n";		  
	}
	else if (number == 3)	//for calculating the area of a triangle
	{
		//prompt for and read the base and height of the triangle
		cout << "Enter the base of the triangle in inches: ";
		cin >> base;
		cout << "Enter the height of the triandgle in inches: ";
		cin >> height;

		//Call the function to calculate the area of a triangle, and display the area
		cout << endl << "Area of triangle is " << AreaOfTriangle(base,height) << "square inches.\n\n";
	}
	else 
	     cout << "Invalid number entered. \n\n"; //number other than 1 and 2 entered


	_getch();	//to hold the screen for viewing
	return 0;
}


//-----------------------------------------------------------------------------------		
double AreaOfRectangle(/* in */ double length,  //length of rectangle
			 /* in */ double width )   //width of rectangle
{
	double areaRect;	//area of rectangle

	//calculate the area
	areaRect = length * width;

	return areaRect;
}

//-------------------------------------------------------------------------------------

double AreaOfCircle(/* in */ double radius )   //radius of circle
{
	const double PI = 3.14159;
	double areaCirc;   //area of circle

	//calculate area of circle
	areaCirc =  PI * pow(radius,2.0);  //note the use of the predefined function pow
	
	return areaCirc;
}

//--------------------------------------------------------------------------------------
double AreaOfTrianglw(/* in */ double base, //length of base of triangle
					  /* in */ double height) //height of triangle
{

	double areaTri;	//area of triangle

	//calculate area of triangle
	areaTri = (base * height) / 2;

	return areaTri;
}

Thanks!

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