p.23 問2

(1)

$$ \begin{array}{l} \llap{2\ )} \ 120 \\ \hline \llap{2\ )} \ 60 \\ \hline \llap{2\ )} \ 30 \\ \hline \llap{3\ )} \ 15 \\ \hline \ 5 \end{array}$$

$$ {\rm 答\hspace2ex}120=2^3\times 3\times 5 $$

p.23 問3

(1)	$$ \begin{array}{l} \llap{2\ )} \ 20 \\ \hline \llap{2\ )} \ 10 \\ \hline \ 5 \end{array}$$	$$ {\rm 答\hspace2ex}20=2^2\times 5 $$
(2)	$$ \begin{array}{l} \llap{2\ )} \ 54 \\ \hline \llap{3\ )} \ 27 \\ \hline \llap{3\ )} \ 9 \\ \hline \ 3 \end{array}$$	$$ {\rm 答\hspace2ex}54=2 \times 3^3 $$
(3)	$$ \begin{array}{l} \llap{2\ )} \ 126 \\ \hline \llap{7\ )} \ 63 \\ \hline \llap{3\ )} \ 9 \\ \hline \ 3 \end{array}$$	$$ {\rm 答\hspace2ex}126=2 \times 3^2 \times 7 $$