Math Is Fun Forum
  Discussion about math, puzzles, games and fun.   Useful symbols: ÷ × ½ √ ∞ ≠ ≤ ≥ ≈ ⇒ ± ∈ Δ θ ∴ ∑ ∫ • π ƒ -¹ ² ³ °

You are not logged in.

#1 2020-05-22 05:42:12


Geometry Proofs

A square has 2 diagonals. (proofs)

A. given
B. unfounded
C. definition of an octagon
D. The number of diagonals is (n-3)n/2, where N is the number of size, which is four for a square
E. 1267200 inches
F. definition of supplementary angles

#2 2020-05-22 21:00:47

bob bundy
Registered: 2010-06-20
Posts: 8,829

Re: Geometry Proofs

hi Ava

Welcome to the forum.

Is that really a question you have been set?  Just multi choice with those alternatives.

Instead I'll try to teach you something useful.

Let's say a convex polygon has N sides.  By convex I mean that the diagonals are always inside the shape.

From one vertex you can make N-3 diagonals. N, less 1 for the point itself, less 2 for the neighbouring vertices as joining the point to those doesn't make a diagonal.

As there are N vertices you can make N times (N-3) diagonals like this … but … every diagonal is counted twice, once A to B, and then again, B to A.  So divide by two to eliminate repeats.

So the formula for the number of diagonals in any convex polygon is

where N is the number of vertices.


Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob Bundy smile


Board footer

Powered by FluxBB