If the lengths of the sides of a triangle are denoted by a, b, and c, then area of triangle is given by area = sqrt(S(S - a)(S - b)(S - c)) where, S = ( a + b + c ) / 2

#include<stdio.h>
#include<conio.h>
#include<math.h>

float ar(float, float, float);
int main()
{
	float area, a, b, c;
	printf("Enter the sides of the triangle : ");
	scanf("%f%f%f", &a, &b, &c);
	if (a + b >c && b + c >a && a + c>b)
	{
		area = ar(a, b, c);
		printf("\n\nArea of the triangle : %f", area);
	}
	else
		printf("\n\nThe triangle you entered is invalid.");
	getch();
	return 0;
}
float ar(float a, float b, float c)
{
	float area;
	float S = (a + b + c) / 2;
	area = sqrt(S*(S - a)*(S - b)*(S - c));
	return (area);
}