[R] predicting x values in a polynomial regression

[Sebastian Leuzinger]

Dear R

I would like to predict x-values in a 4th order polynomial regression:

x <- c(1:10)
y <- c(2,7,19,49,89,94,97,98,92,89) # these are percentages

lm(y ~ x+I(x^2)+I(x^3)+I(x^4)-1) -> lm1

now I would like to know what the model fit (x-value) for y=50 is. This 
results in solving a 4th order quadratic equation. polyroot() does not 
really help because it only gives me the x-values for y=0. I have tried 
with nls() which sort of works, but I am sure there is a much easier 
solution to that, can anyone give me a hint?
-- 
Sebastian Leuzinger
Institute of Botany, University of Basel
Schönbeinstr. 6 CH-4056 Basel
Ph. 0041 (0) 61 267 3511
fax  0041 (0) 61 2673504
email: Sebastian.Leuzinger at unibas.ch <mailto:Sebastian.Leuzinger at unibas.ch>
web:  http://www.unibas.ch/botschoen/leuzinger/e.shtml 
<http://www.unibas.ch/botschoen/leuzinger/d.shtml>