# Definitions of Polygons

1. Aug 13, 2011

I'm working through the following book: Principles of Mathematics, by Allendoerfer & Oakley.

Since I haven't taken a proof-based course yet, and won't be able to until spring 2012 :grumpy:, I want to make sure that I'm not forming habits that will hurt me when I do. There are some answers that aren't provided in the back of the book, so I want to check them with you all.

On Page 5, Problem 2:

1. The problem statement, all variables and given/known data

Assume that polygon, side of a polygon, angle, length of side, equal, and parallel, have been previously defined. Then define:

a) Parallelogram.
b) Rhombus.
c) Pentagon.
d) Regular Pentagon.
e) Trapezoid.
f) Hexagon.

2. Relevant equations

I believe the exercise wants to ensure that I'm using the "if and only if" bi-conditional logical connective correctly. My understanding is that the "if" includes all of the cases that follow the next clause, and "only if" excludes all others. So, I need to make sure that my definitions do not include other polygons.

3. The attempt at a solution

Parallelogram: A four-sided polygon is a parallelogram if and only if it has two sets of parallel sides.

Rhombus: A four-sided polygon is a rhombus if and only if its sides are equal and has two pairs of congruent angles.

Pentagon: A polygon is a pentagon if and only if it has 5 sides.

Regular Pentagon: A polygon is a regular pentagon if and only if it has 5 equal sides.

Trapezoid: A four-sided polygon is a Trapezoid if and only if it has two parallel sides and two pairs of congruent angles.

Hexagon: A polygon is a hexagon if and only if it has 6 sides.

Thank you for your help.

Edit: I have avoided simply looking up the definitions on the internet so that I'm given the chance to reason my way to an answer, if possible.

Last edited: Aug 13, 2011
2. Aug 13, 2011

### I like Serena

Just a few comments on your statements (mostly nitpicking, but that is what math is about ).

In a rhombus the sides are not equal (that would be pretty weird!), but the lengths of the sides are equal.

The two pairs of congruent angles are redundant in a rhombus (but not wrong).

A pentagon with 5 sides of equal length does not have to be regular.
Perhaps you could try to find an example?

A trapezoid does not have to have two pairs of congruent angles.
Perhaps you could look up the definition of a trapezoid?

The rest looks good!

3. Aug 13, 2011