Giải thích các bước giải:
Gọi $UCLN(2n+3,2n+5)=d$
$\rightarrow 2n+3\quad\vdots\quad d$
$2n+5\quad\vdots\quad d$
$\rightarrow 2n+5-(2n+3)\quad\vdots\quad d$
$\rightarrow 2\quad\vdots\quad d$
$\rightarrow d=1$ do $2n+3,2n+5$ lẻ
$\rightarrow BCNN(2n+3,2n+5)=(2n+3)(2n+5)\rightarrow đpcm$