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I have a quadrangle which sides consist of parts of rays, and I only know the coordinates of two points on each ray.

I need to determine if a point $(x,y)$ lies in such quadrangle.

In this picture, I painted the sides of the quadrangle red in case you don't understand about what quadrangle I'm talking.

Lee David Chung Lin
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Kedel Mihail
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  • Can you determine the intersection points of the rays? – Dr. Mathva Apr 06 '19 at 18:04
  • I multplied LHS of their equations as following. Can you check the sign of this product? And the logic? $$(\frac{x}{5}+ \frac{y}{3}-1).(\frac{x}{4}+ \frac{y}{-7}-1).(\frac{x}{-8}+ \frac{y}{-12}-1).(\frac{x}{5}+ \frac{y}{-2}-1) $$ – Narasimham Apr 17 '19 at 13:51

1 Answers1

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Let $A(x_a, y_a), B(x_b, y_b), C(x_c, y_c)$ and $D(x_d, y_d)$ form the quadrilateral $ABCD$ with area $S$ and let $P$ be the point you've chosen.

Calculate the areas of the triangles $\triangle PAB, \triangle PBC, \triangle PCD$ and $\triangle PDA$, for instance, with the shoelace formula. Let their sum be $S'$. Then

\begin{align*} S'=S&\implies P\text{ lies inside }ABCD\\ S'>S&\implies P\text{ lies outside } ABCD \end{align*}

Dr. Mathva
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