nls.fit<-nls(ch8 ~ e0 + (emax * dose)/(dose + ed50), 
start = c(ed50=25,e0=5,emax=20), control = nls.control(
maxiter = 100),trace=T,na.action=na.omit,data=edd)

summary(nls.fit)



ffit = function(doses =c(0,100), ed50=75.76,e0=1.885,emax=15.85,lambda=0.75,pl=T,...) {
dose = seq(doses[1],doses[2],length=100)
yy=e0 +(emax * dose^lambda)/(dose^lambda+ed50^lambda)
if (pl) plot(dose,yy,type="l",...) else lines(dose,yy,...)
}