Hey, new here, and I have a question

Hey I found this site by looking for help with this problem.

If a polygon has exactly 65 diagonals, how many sides must it have.

What I know now...
n(n-3)/2

thats a divison thing n(n-3) divided by 2. That's the formula for finding the exact amount of diagonals in a certain sided shape.

1 2 3 4 5 6 n
X X 0 2 5 9 N(N-3) / 2

I know this has to be reversed, but it just hurts my head to think about it.

Thanks for the help in advance... This is a very good idea for use of bandwith!

Then

n(n-3)/2 = 65



n(n-3) = 130

n^2 -3n - 130 = 0

solve for n

Where did you get all that crappy stuff, IV?

"If a polygon has exactly 65 diagonals, how many sides must it have?"
65, of course.

Why don't you draw a circle, insert a 5sided-gon in it, join the center
of the circle to each of the gon's 5 corners...OK???

...if that's too hard, make it a 4sided-gon: a square.

How many different lines can you draw joining the vetices of a regular hexagon? It's nine isn't it

0,2,5,9,14,20,27,35,44,54,65,77,90,104,119,135,

So starting with triangles, (n=3) this would be the number of diagonals of the firsty few n-gons.

Denis said:

"If a polygon has exactly 65 diagonals, how many sides must it have?" 65, of course.

Click to expand...

Really? A square (four sides) has two. A pentagon (five sides) has five. A hexagon (six sides) has nine. And so forth.

Eliz.

stapel said:

Denis said:

"If a polygon has exactly 65 diagonals, how many sides must it have?" 65, of course.

Click to expand...

Really? A square (four sides) has two. A pentagon (five sides) has five. A hexagon (six sides) has nine. And so forth. Eliz.

Click to expand...

Guilty as charged
I somehow was thinking of a diagonal as a line from centre to a corner;
guess my head was still throbbing from listening to them 20 bingo women :?