Help Needed by Maths literate person

[sAsLEX]

punch it through MATLAB if i still remember how to use damn program.

in a strong cockney accent * MATLAB its DEAD EASY* for all the AU enginners!

[Brian d marge]

As nice as the data plot is the raw data would be far more useful.

I am on to that will post the raw data as soon as I get my hands on it ....

The simpsons rule ,now that would give me the area easy enough

( something like ...hieght/3 { Yo +4Y1+Y2.......}=area

Thats easy ,,,cool ...but I should be able to work backwards to find the equation ..

Lets say for exaple I use simpsons rule and find the area ..is 16.7 whamajamas ..

Using an old copied example I found

Area = integral between 3 and 1 ( X^2 +4)dx
= { X^3/3+4X}
= (3^3/3+4 x 3)-( 1^3/3+4 x 1 )=16.7

I am answering my own question here ,,,I can see it ..

Anyway I will carry on ...so yes From the area I should be able to go back to the equation ,,,,,

and with that equation I should then be able to add and subtract mutiply equations together ,,,

Anyway I will post the data as soon as I get my mitts on it ,,,,,,,
see how we go

Stephen

[Brian d marge]

Ok
when the raw data turns up Ill post it ...till then ..try this see if I am in the ball park

Take a parabola ...X^2 cutting the X axis at -2 and 2 and the Y axis at 4

When Y = 0
x = -2 and 2

( X-2) (X+2)
= 4-X^2
Intergrate between -2 and 2 to get the area I get 10 2/3

checked with simpsons Rule get the same ....which is cool ..and exactly what I wanted great ..

Now the original graph as was pointed out was a sine function SOOOOOOO all I have to do is expand the above to include ,,,the sine function ????


So like a plan??????


Stephen

BTW thanks for all the help

[zooter]

There are common integrals listed in the back of your logarithm tables which should see you right.

[erik]

I don't actually know the proper way to do this kinda stuff.

But, from plotting the data in excel and messing around with sine equations, the closest I've got to an equation that gives a similar curve is:

=0.316*SIN(RADIANS(A1-11))-0.316/2*SIN(RADIANS(A1-11))+0.158

Take a look at the graph in the excel file. There is the original data curve, then the approximation based on the above equation. And I've plotted the difference between the two. If you could figure out an equation for the difference between the two and add it to the other approximation, you'd get the equation you wanted. 'course, that's just as hard or harder than finding the equation for the original data...

Dunno if this helps or not.

I don't think you'll have much luck with a polynomial equation or parabola. I think it has to be based on trig functions like sine and cos etc.

[erik]

Actually, the trick might be combining a parabola plus a sinewave.

I spent some time with excel again, the parabola+sinewave curve matches more closelly than the previous sinewave only.
Maybe with some more fiddling with the figures, you could get it an even closer match.
Or maybe there's some other easier way to do it...

[parsley]

I suppose the proper way of getting the equation of a curve is to use Fourier analysis, but I've always preferred Bezier curves. I've used them quite a lot for curve editing functions in GUIs, but they're also a simple way of defining a curve.

[k14]

Nah just fit a 6 order poly line of best fit to teh line in excel, gets a pretty damn close fit then. That gives a formula of y = 1E-10x6 - 2E-08x5 + 2E-06x4 - 0.0001x3 + 0.0025x2 - 0.0095x + 0.0082

Then you integrate it and fine the finite integral from the lower to the upper limit (what were they again?). Not too hard to do. I'll do it later if you want.