source("http://192.38.117.59/~linearpredictors/datafiles/readBirthWeight.R")

## model with product term
model <- lm(log10(bw) ~  log10(ad) + log10(bpd) + I(log10(ad)*log10(bpd)),
            data = secher)

## function to calculate the predicted line for any value of ad.
f <- function(ad){
  list(
       ad = ad,
       log10ad = log10(ad),
       Intercept = model$coef["(Intercept)"] + log10(ad)*model$coef["log10(ad)"],
       Slope = model$coef["log10(bpd)"] + model$coef["I(log10(ad) * log10(bpd))"]*log10(ad)
       )
}

sapply(c(90,100,110), f)