- #1
srfriggen
- 306
- 5
Hello all,
I've been trying to work out this problem I came across yesterday when a professor mentioned it in a math education course. It states, "Take a circle and put two dots on the circle then connect them with a cord. How many sections of area does the cord split the circle into?" Of course the answer is 2.
Now do it with 3 dots. The answer is 4.
Now do it with 4 dots. The answer is 8.
Now do it with 5 dots. The answer is 16.
How many if there are 6 dots?
One might think, naively as I did, that the formula for the number of sections is 2n-1, so 6 dots should have 32 sections. But it actually has 31. That was the point of the lesson.
We never got the formula so I've been trying to figure one out.
I decided to look at the number of dots and how many cords they form, and I was able to find a recursive definition and conjecture a formula. I proved it using induction.
I will write the information out like this: # of dots / #of cords;
1 / 0
2 / 1
3 / 3
4 / 6
5 / 10
6 / 15
7 / 21
8 / 28
The recursive series can be understood as s(n)=s(n-1) + (n-1), and the explicit formula is:
s(n)=(n-1)+(n-2)+...+ 3+2+1.
Now I have a nice relationship between dots and cords. Next I will show you the table of dots to cords to sections, which I worked out manually up to n=7.
1 / 0 / 1
2 / 1 / 2
3 / 3 / 4
4 / 6/ 8
5 / 10 / 16
6 / 15 / 31
7 / 21 / 58
8 / 28 / ?
So this is where I am stuck. I am trying to find a relationship with the number of sections and the number or cords or dots. I do not see any obvious recursive relationship, and certainly do not see any obvious formula.
Perhaps a different set of eyes/synapses on the problem would shed more light. Is there a more systematic approach I can use, rather than jus trying to "see" it?
I've been trying to work out this problem I came across yesterday when a professor mentioned it in a math education course. It states, "Take a circle and put two dots on the circle then connect them with a cord. How many sections of area does the cord split the circle into?" Of course the answer is 2.
Now do it with 3 dots. The answer is 4.
Now do it with 4 dots. The answer is 8.
Now do it with 5 dots. The answer is 16.
How many if there are 6 dots?
One might think, naively as I did, that the formula for the number of sections is 2n-1, so 6 dots should have 32 sections. But it actually has 31. That was the point of the lesson.
We never got the formula so I've been trying to figure one out.
I decided to look at the number of dots and how many cords they form, and I was able to find a recursive definition and conjecture a formula. I proved it using induction.
I will write the information out like this: # of dots / #of cords;
1 / 0
2 / 1
3 / 3
4 / 6
5 / 10
6 / 15
7 / 21
8 / 28
The recursive series can be understood as s(n)=s(n-1) + (n-1), and the explicit formula is:
s(n)=(n-1)+(n-2)+...+ 3+2+1.
Now I have a nice relationship between dots and cords. Next I will show you the table of dots to cords to sections, which I worked out manually up to n=7.
1 / 0 / 1
2 / 1 / 2
3 / 3 / 4
4 / 6/ 8
5 / 10 / 16
6 / 15 / 31
7 / 21 / 58
8 / 28 / ?
So this is where I am stuck. I am trying to find a relationship with the number of sections and the number or cords or dots. I do not see any obvious recursive relationship, and certainly do not see any obvious formula.
Perhaps a different set of eyes/synapses on the problem would shed more light. Is there a more systematic approach I can use, rather than jus trying to "see" it?