for similar shapes, their corresponding sides will form the same proportion or ratio for any pair of corresponding sides, now, let's check the ratio of each corresponding side on these two.
[tex]\bf \cfrac{\textit{small triangle}}{\textit{large triangle}}\qquad \qquad \cfrac{12}{45}\implies \cfrac{4}{15}~\hfill \cfrac{25}{75}\implies \cfrac{1}{3}~\hfill \cfrac{15}{36}\implies \cfrac{5}{12} \\\\[-0.35em] \rule{34em}{0.25pt}\\\\ ~\hfill \stackrel{\textit{the triangles are \underline{not} similar}}{\cfrac{4}{15}\ne \cfrac{1}{3}\ne \cfrac{5}{12}}~\hfill[/tex]
