Chapter 2, Problem 19PE

(Geometry: area of a triangle) Write a program that prompts the user to enter three points $\left(\text{x1,}\text{y1}\right)\text{,}\left(\text{x2,}\text{y2}\right)\text{,}\left(\text{x3,}\text{y3}\right)$ of a triangle and displays its area. The formula for computing the area of a triangle is

$\begin{array}{l}s=\left(side1+side2+side3\right)/2;\\ area=\sqrt{s\left(s-side1\right)\left(s-side2\right)\left(s-side3\right)}\end{array}$

Here is sample run: