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Anyone good with Excel?!?!?!?

Old 05-05-09, 06:05 PM
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Anyone good with Excel?!?!?!?

Hey guys I am looking for some help with excel. I will explain what I am doing...

I have an XY scatter plot for three different sets of data. I am adding a best fit line (trendline) for each set. Best fit is a polynomial order of 2. Everything is okay except it is giving me the equations as

y = 3E-05x^2 - 0.0004x

and

y = 3E-05x^2 - 0.0005x

I want to force out the linear part and just have it as

y = cx^2

How can I go about doing this? Thanks!
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Old 05-05-09, 08:19 PM
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I think you are missing something, this is an algebra problem, not Excel
(but you could use Excel to solve for x)

Since both equations = y; then:

3E-05x^2 - 0.0004x = 3E-05x^2 - 0.0005x

then subtract 3E-05x^2 from both sides to get:

- 0.0004x = - 0.0005x

The only value of x where that works is x = 0

I would need to see the original data to fit a y = cx^2 type curve,
which I would be happy to do, but it may be a day or two before I can get to it.
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Old 05-05-09, 08:29 PM
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Right-click on the trendline, select "Format Trendline" select Options, select a linear trendline instead of polynomial.

edit, sorry that's Type, not Options
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Old 05-06-09, 05:12 AM
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Originally Posted by byte_speed View Post
I think you are missing something, this is an algebra problem, not Excel
(but you could use Excel to solve for x)

Since both equations = y; then:

3E-05x^2 - 0.0004x = 3E-05x^2 - 0.0005x

then subtract 3E-05x^2 from both sides to get:

- 0.0004x = - 0.0005x

The only value of x where that works is x = 0

I would need to see the original data to fit a y = cx^2 type curve,
which I would be happy to do, but it may be a day or two before I can get to it.

It is in excel. The two formulas are for two different sets of data so they are not equal.

Originally Posted by valygrl View Post
Right-click on the trendline, select "Format Trendline" select Options, select a linear trendline instead of polynomial.

edit, sorry that's Type, not Options
Unfortunately I need to do a polynomial best fit. The data I am graphing is force of drag versus wind velocity. I can not get an accurate best fit with a linear trendline.
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Old 05-06-09, 07:12 AM
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Originally Posted by Jynx View Post
It is in excel. The two formulas are for two different sets of data so they are not equal.



Unfortunately I need to do a polynomial best fit. The data I am graphing is force of drag versus wind velocity. I can not get an accurate best fit with a linear trendline.
Ok, it's been a long time and I don't remember the terminology well..

but I think a polynomial equation is
y = ax^2 + bx + c

and if you try to get rid of the "bx" + c part you're just going to have a parabola.

but if you want it to force that, you have to create a separate data range that contains data that expresseses the equation you want to graph, and graph it. - I found this by googling "graph equation in excel" http://peltiertech.com/Excel/ChartsH...tEquation.html
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Old 05-06-09, 10:14 AM
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Originally Posted by valygrl View Post
Ok, it's been a long time and I don't remember the terminology well..

but I think a polynomial equation is
y = ax^2 + bx + c

and if you try to get rid of the "bx" + c part you're just going to have a parabola.

but if you want it to force that, you have to create a separate data range that contains data that expresseses the equation you want to graph, and graph it. - I found this by googling "graph equation in excel" http://peltiertech.com/Excel/ChartsH...tEquation.html
Thank you. I figured it out.
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