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Root command
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Please have a look at the attached file. The command Root[f, 90, 110] gives undefined as result, but Root[f, 60, 140] and Root[f, 99, 101] gives 100.
Is this a bug? Any other explanation?
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It's using a numerical algorithm so is probably having trouble with the big numbers. You can use the CAS View for exact answers, or this seems more stable:
f(x) = 10x² + 1100x  10000
Root[f(x) / 1000, RandomBetween[50, 99], RandomBetween[101, 150]]
Also try:
Root[f]
which is designed for Polynomials, Root[f, a, b] is designed for more general functions
The actuall function was given at one of the National Exams in Norway, and the pupils want to restrict the domain to positive x values. They use the commands Function[...] or If[...] to restrict the domain. Then the command Root[f] does not work, and they need to use one of the numerical variants.
Simply use twice
Root[f, <StartingValue>]
as per att'd sheet.
https://ggbm.at/568825
I know the command Root[f, <StartingValue>], and that one works fine. But why are not Root[f, a, b] working for a=90 and b=110. I think you should have a look at the code in GeoGebra to see if there is something strange in there... According to the GeoGebra manual, Root[f, a, b] uses the Regula Falsi method. I cannot see any reason why this method should not work for a simple quadratic function like the one in my example. (I can understand that you are questioning the use of this method on a simpel function like this, but the command should work anyway.)
my idea is that Geogebra algorithms work properly, numerically speaking, with NewtonRaphson method (only one starting value needs), despite what the manual says. In fact, I never met discrepancies by using the root searches in such a manner. It's my thought, of course, coming from a long use of Geogebra. Cheers
Philippe
Hi, to play with regula falsi method with this concave functtion on [90;110]
https://ggbm.at/568831
It hasn't said that for a while (as it's not true any more): Root
Anyway, we've improved it for 5.0.20.0 so all answers from Root[f, a, b] will generally be more accurate now so thanks for the report :)
Thank you!
It is a big job to keep the documention in sync with the code, and an even larger job to keep the translations of the documentation in sync. I read this in the Norwegian manual, and that one needs to be updated. (We are working on it, but it is a huge job to track all the updates...)
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