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## little bit of simple math

This is a discussion on little bit of simple math within the General Off Topic+ forums, part of the QJ.net Forum Miscellaneous category; Ok, i have to figure out an equation, I have solved it and it is not working so i hope ...

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1. ## little bit of simple math

Ok, i have to figure out an equation, I have solved it and it is not working so i hope you can help

I need an equation for the x value of the intercept between the perpenicular lines which pass through the midpoint of 2 chords (which will give me the x value of the center of the circle).

the end result should be x=

it is really hard to post what I got but ill try anyways:
Spoiler for equation:
X=(-(((x1-x2)/(y1-y2))((x1+x2)/2))-((y1+y2)/2)+(((x1-x3)/(y1-y3))((x1+x3)/2))+((x1+x3)/2))/(-((x1-x2)/(y1-y2))((x1-x3)/(y1-y3))

bold is the numerator

2. I know how to do that, but it looks big.

So I can't be arsed

3. woops, looks like my equation does work, it was just that i was using exact points on the circle, which canceled out the bottom, making it 0 thus giving me an undefined answer. this shouldnt happen in experimental data.

but now i need one more thing...

how do you solve this (or at least set it up, i know it has to do w/ factorials but forgot the equation )

from 5 points. each one of those points needs to be paired up w/ another point not itself once. from those combos, each combo has to be paired up w/ every other combo(once) not itself. how many total combinations is that?

incase that wasnt clear, here is a short example of what i mean using 4 points.
Spoiler for example:
points:a,b,c,d
ab
ac
bc
bd
cd

ab-ac
ab-bc
ab-bd
ab-cd
etc...

4. wow i respect you guys for know how to make that

5. I'm not sure if you explained it better I might be able to help you. What math course are you taking?

6. its not for a math course, it is for a science course. I needed to make an equation to determine the center of a circle that a stream of electrons make when bent with a magnetic field.

why i needed what i needed in the first equation:
those points each make a chord. and there is a therom which says that in EVERY chord, the perpindicular bisector passes through the center of the circle. now if you have 2 chord, their perpindular bisectors will therotically intersect in the center of teh circle.

the only problem is that the slopes are so small, even a slight inaccuracy can totally throw off the center of the circle. so that is why i need EVERY possible combination, take out the outliers and average them to find the approximate center. to average them, i need to know how many combinations total there are for 5 points. I wont do this by hand of course, that is what computers are for. but if someone could give me that equation,

it was something like 5!/2!(5!-1!) (for finding how many combinations of points there are) and that same equation can be reused to find how many combination of lines those points make.
but i dont think that equation is it.

7. Ok.... That's a little over me.

8. ok i got it, it is n!/(n-2)! which gives me 830 or so combinations. but my calc battery ran out right after i wrote the program so i couldnt test it. but it will take like 6 minutes to find one circle center (i will be doing multiple tests) so this might be a little impracticable. i might just make it on the computer...

9. Ummm... the anwser is 2. :o

jk :P

tbh, I could probably understand that if someone explained it to me. Geometry's where I'm really at in maths though. I hate all that pure mathematics shizzle though...

11. for you i will try to explain in geometry terms:

you know from geometry, that every chord is perpendicular to the center. At the midpoint, a line which is perpendicular to the chord will pass through the center of the circle. if you have two chords, the intercept between the chord will be the center of the circle.

so you start out with the equation of the chord:
Y=((y1-y2/x1-x2)-x1)* x + y1
and Y=((y3-y4/x3-x4)-x3)*x + y3
now you need to replace the slope with the opposite recriprocal, and the point substitution with the midpoint, and set them equal to each other and solve for x

12. In fact, I change my mind on understanding... call back in about two years time...

I sort of get it. But not that much.

13. OMG we 1st year highschool people are just studying elementary algebra...

14. dracule watf.

its simple. i dont think theres need for complex equations.

think of it as a mass spectrometer

a magnetic field B is directed in or out of the paper and a stream of particles is shot out into the magnetic field. since the magnetic field only changes direction then they curve in a circle and that is moddeled by

m = ((er^2)/2V)B^2

or

another equation (im not sure if the above pertains to you) where centripital force is the force that makes the electrons bend

r=(mv)/(qB)

15. but in order to you need to know the velocity of the thing, which is what we are trying to find, along w/ the radius wo we know how much force is being. we know the voltage proppelling the electron forward, and the amps of the magnets surrounding, and a graph to get the curvature of the stream.

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