a.

$\left({\left.{\begin{array}{*{20}c}1&4\\2&9\\\end{array}}\right|\begin{array}{*{20}c}1&0\\0&1\\\end{array}}\right)\to\to\left({\left.{\begin{array}{*{20}c}1&4\\0&1\\\end{array}}\right|\begin{array}{*{20}c}1&0\\{-2}&1\\\end{array}}\right)\to\left({\left.{\begin{array}{*{20}c}1&0\\0&1\\\end{array}}\right|\begin{array}{*{20}c}9&{-4}\\{-2}&1\\\end{array}}\right)$

b.

$ \left( {\begin{array}{*{20}c} 1 & 4 \\ 2 & 9 \\ \end{array} } \right) \cdot \left( {\begin{array}{*{20}c} 9 & { - 4} \\ { - 2} & 1 \\ \end{array} } \right) = \left( {\begin{array}{*{20}c} 1 & 0 \\ 0 & 1 \\ \end{array} } \right) $

Met de GR