b
(b) $|{A_i}| = \left| {\,\begin{array}{*{20}{c}}{{a^i}}&{{b^i}}\\{{b^i}}&{{a^i}}\end{array}\,} \right|$ = ${({a^i})^2} - {({b^i})^2}$, $|a|\, < 1,|b|\, < 1$
$\sum\limits_{i = 1}^\infty {|{A_i}|} = ({a^2} - {b^2}) + ({a^4} - {b^4})$$ + ({a^6} - {b^6}) + .......$
$ = ({a^2} + {a^4} + {a^6} + ......)$$ - ({b^2} + {b^4} + {b^6} + .......)$
$ = \frac{{{a^2}}}{{1 - {a^2}}} - \frac{{{b^2}}}{{1 - {b^2}}}$
$ = \frac{{{a^2} - {a^2}{b^2} - {b^2} + {a^2}{b^2}}}{{(1 - {a^2})(1 - {b^2})}}$
$ = \frac{{{a^2} - {b^2}}}{{(1 - {a^2})(1 - {b^2})}}$.