(Geometry: area of a triangle)
Write a C++ program that prompts the user to enter three points (x1, y1), (x2, y2), (x3, y3) of a triangle and displays its area.
The formula for computing the area of a triangle is:
s = (side1 + side2 + side3) / 2
area = square root of s(s - side1)(s - side2)(s - side3)

Sample Run:
Enter three points for a triangle: 1.5 -3.4 4.6 5 9.5 -3.4
The area of the triangle is 33.6