I want to add a note here. It seems to me that if you apply the chain rule, you should get this:

d(f(tx))/dx[i] =
{df(tx)/dtx[i]} * dtx[i]/dx[i] =
{df(tx)/dtx[i]} * t

In other words, the denominator of the first factor to the right of the = sign just above P.2 should be dtx[i] instead of dx[i].

I know the text is right, because I checked another book. I just don't understand why.