correct and enhance ch3 applied questions

relative error in 8aii) was referring to independent var
12b) and 12c) needs +0 to fit through origin
Added a new example to 12c) to illustrate other situations this fit
could be generated
13a) chas is a factor variable, so translated it to one. This affects
whether it is significant
15b) no need to ignore the independent var, this is done automatically