[R] Fitting my data to a Weibull model

[Marcio]

Hi guys,
I have a data-set that fits well into a Weibull model y = a-b*exp(-c*x^d).
I want to estimate the parameters of the coefficients a, b, c and d, given x
and y.
Can you guys help me?

Just as an example, I fit the data
y <- c(1,2,3,4,10,20)
and 
x <- c(1,7,14,25,29,30)

According to this model above, and using the software CurveExpert, and I got
the estimates of a (2.95),b (2.90),c (2.82e-2) and d (2.97).

This is just a sample data but I would like to do it on R because I can loop
multiple times through all my files.
 
I would also like to fit a logistic model y = a/(1+b*exp-c*x)

Can you help  me????
Thanks

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