\(ΔABC\backsim ΔDEF\) theo hệ số tỉ lệ \(k\)
\(→\begin{cases}\dfrac{AB}{AC}=\dfrac{DE}{DF}=k\\\dfrac{AB}{BC}=\dfrac{DE}{EF}=k\\\dfrac{AC}{BC}=\dfrac{DF}{EF}=k\end{cases}\\↔\begin{cases}\dfrac{DF}{DE}=\dfrac{AC}{AB}=\dfrac{1}{k}\\\dfrac{EF}{DE}=\dfrac{BC}{AB}=\dfrac{1}{k}\\\dfrac{EF}{DF}=\dfrac{BC}{AC}=\dfrac{1}{k}\end{cases}\)
\(→ΔDEF\backsim ΔABC\) theo hệ số tỉ lệ \(\dfrac{1}{k}\)