[R] solving x in a polynomial function

[Jeff Newmiller]

help.search("polynomial")
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Sent from my phone. Please excuse my brevity.

Mike Rennie <mikerennie.r at gmail.com> wrote:

>Hi there,
>
>Does anyone know how I solve for x from a given y in a polynomial
>function? Here's some example code:
>
>##example file
>
>a<-1:10
>
>b<-c(1,2,2.5,3,3.5,4,6,7,7.5,8)
>
>po.lm<-lm(a~b+I(b^2)+I(b^3)+I(b^4)); summary(po.lm)
>
>(please ignore that the model is severely overfit- that's not the
>point).
>
>Let's say I want to solve for the value b where a = 5.5.
>
>Any thoughts? I did come across the polynom package, but I don't think
>that does it- I suspect the answer is simpler than I am making it out
>to be. Any help would be welcome.