We have:
[tex]ab=8a(1)[/tex][tex]b+d=20(2)[/tex]In order to prove that d=12, we do as follows:
[tex]ab=8a\Rightarrow b=\frac{8a}{a}\Rightarrow b=8(3)[/tex]Now, replacing (3) on (2), we have:
[tex]8+d=20\Rightarrow d=20-8\Rightarrow d=12(4)[/tex]And is proven that d=12.
