# Right Angled Triangle

Hello,

I have come across a problem that I don't have an elegant solution for...

The question is: Given a square 4 x 4 grid of dots, how many right angled triangles can you create by selecting three dots?

You could of course use a brute force method. I have considered permutations / combinations, but ran into some difficulties of eliminating solutions. I've considered approaching it through a symmetrical approach as well.

Any help on the problem will be very helpful! Note by Jae Joon Chang
1 month, 1 week ago

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@Jae Joon Chang - Here is the solution-

Basically what we are looking for is the number of sets of 3 points within 16 points minus the sets of 3 points that lie on the same line, or in other words are collinear.

The number of sets of 3 points within the 4x4 grid is $C^{16}_{3} = 560$

Now, on each vertical and horizontal line, there are 4 ways that you can choose 3 collinear points (Proof given at the end)

There are 4 vertical lines, and 4 horizontal lines, so the number of sets here is $4 \times 4 \times 2 = 32$

The grid has two main diagonals, and four minor diagonals (Shown in an image at the end)

Again, there are 4 ways that 3 collinear points can be chosen on the main diagonals, and 1 way on each of the minor diagonal lines.

This means, that the number of sets on the diagonals is $2 \times 4 + 4 = 12$

Thus, the answer is $560-32-12 = \boxed{516 \text{ triangles}}$

Images - These are the 4 ways you can choose a set of 3 collinear points on a line The green lines are the minor diagonals

Sorry for bad artistic skills :)

- 1 month, 1 week ago

@Percy Jackson - thanks for your solution. I think the 516 triangles are simply triangles, whereas the question wanted to find the number of potential right angled triangles you can get.

A friend of mine ran a C++ programme finding three points that satisfies Pythagoras' Theorem and it returned with 200 right angled triangle.

I like the idea of eliminating collinear points, but I'm not too sure on how you could look to eliminate points that create a scalene triangle.

- 1 month, 1 week ago

Oh! Right angled triangles! I missed that part. Yes, then it would be difficult to eliminate all other triangles which are not right angled...Can you share the C++ code with me if its possible?

- 1 month, 1 week ago

apologies for the formatting, but this is what my friend had put in.

IN: import itertools

IN: def is_right(a,b,c): ab2 = (a-b)(a-b) + (a-b)(a-b) ac2 = (a-c)(a-c) + (a-c)(a-c) bc2 = (b-c)(b-c) + (b-c)(b-c) s = sorted((ab2, ac2, bc2)) return (s + s == s and a != b and a != c and b != c)

IN: def righttriangles (x, y): return {frozenset((a, b, c)) for (a, b, c) in itertools.product( itertools.product(range(x), range(y)), itertools.product(range(x), range(y)), itertools.product(range(x), range(y)) ) if isright(a, b, c)}

IN: len(right_triangles(4, 4))

- 1 month, 1 week ago

ok, thanks!

- 1 month, 1 week ago