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 Math Help > Geometry > Point And Triangle

Here's a challenge for you:

Write an algorithm -- a step-by-step procedure -- to determine whether a point is inside a triangle.

The input to the algorithm is eight numbers: the x,y coordinates of a single point, P and 3 pairs of x,y coordinates giving the corners A, B, and C of the triangle.

The output from the algorithm is the word "Inside" if the point is inside the triangle, or the words "Not Inside" if the point is not inside the triangle.

To see what the algorithm should do, enter the input here:

Point P (,)

Triangle A (,),  B (,),  C (,)

And then look at the output here:

This web page actually contains the algorithm, written in JavaScript.

Try do do the challenge yourself.  Then, if you want to check your answer, look at an explanation of how I met this challenge.  There are examples written in JavaScript, REXX, and even an Excel Spreadsheet!

### Related Pages in this website

Back to Geometry and Trig

Points and Lines -- answers questions like finding the equation of a line given a point and a slope, or another point

Using the determinant of a matrix to calculate the area of a polygon

Triangle Area using Determinant

Triangle Area using Vectors

Pythagorean Theorem

The webmaster and author of this Math Help site is Graeme McRae.