In the proof attached, it says you can substitute the equation for d into equation (17) in and it will yield equation (16). However, d does not appear in (17) so in what sense do they mean I can substitute the expression for d into equation (17)?
In the proof attached, it says you can substitute the equation for d into equation (17) in and it will yield equation (16). However, d does not appear in (17) so in what sense do they mean I can substitute the expression for d into equation (17)?
Yes it is in (17). Look at the line right above that:
$\displaystyle d=-ax_1-by_1-cz_1$ there is the d in different format.