I was in geometry class today when I came across the following formula for the external angle of a regular polygon with n sides:
$$Ea = \frac{360ยบ}{n}$$
So I thought if $$ n\rightarrow\infty $$ then $$ Ea\rightarrow0$$
Thus if a circle is a polygon with an infinite number of sides it's external angles would approach 0.
I then tried to do the reverse way, trying to figure out a way to prove the premise that a circle has n = infinity; however I could not prove it.
In this sense, how do you prove the infinite number of sides in a perfect circle?
Best Answer
In a way, there is no reverse way for you to prove. You must always be careful about what your definitions. That way, you will always have a clear mathematical way of writing what you are trying to prove and how to prove it, as well as how "reverse ways" of some theorems would look.
For example, in your case, you have defined the following:
What you have shown is this:
What you Have not shown is this:
Why haven't you shown this? Well, let's see:
First of all, you haven't defined a circle. Sure, you can define a circle, but a circle will not be a regular polygon, at least not by the definition of a regular polygon. OK, that may be a problem you can overcome. We can say that a circle is some sort of curve, just like a polygon. However, there is another problem:
You did not define what it means for two curves to approach one another. Without defining exactly what it means for a polygon to approach a circle, you cannot say a circle approaches it, neither can you then say "How to prove the inverse of this statement?"
Even if you define what it means for two curves to approach each other, you are still miles away from showing the statement which you, ultimately, want to show:
Again, why could you not proven this statement? Well, it isn't a mathematical statement, so it cannot be proven in a mathematical way. For example, a polygon is defined as a collection of a finite amount of straight lines, so the concept of "a regular polygon with an infinite number of sides" does not exist yet. You can define it, sure, but if you just define it as a circle, then the statement becomes empty. You could define "generalized polygons" as such:
In this case, you must, of course, define what a limit of a curve is, but that is possible (albeit not trivial).
If you decide to define it that way, you can now prove the statement:
This statement is, basically, the statement "$n$-sided polygons approach a circle as $n$ tends to infinity", in mathematical terms. However, notice what happened:
In defining generalized polygons, you lost the ability to speak about the number of sides of a polygon. For example, a square is a $4$ sided polygon and has $4$ sides. You can say "the number of sides of this polygon is such and such". You cannot say the same thing about generalized polygons. You can define the number of sides as such:
If you decide the number of sides that way, then the statement
Becomes equivalent to the statement