$\color{black}{\text{(Hình vẽ minh họa trong ảnh)}}$
$\color{black}{\text{Giả sử}}$ $\color{black}{ΔABC \sim ΔA'B'C'} $
$\color{black}{\to\left\{ \begin{array}{l}\color{black}{\dfrac{AB}{A'B'}=\dfrac{AC}{A'C'}=\dfrac{BC}{B'C'}}\\\widehat{ABC} = \widehat{A'B'C'}\end{array} \right.}$
$\color{black}{\to\left\{ \begin{array}{l}\color{black}{\text{Đặt}}\color{black}{\dfrac{AB}{A'B'}=\dfrac{AC}{A'C'}=\dfrac{BC}{B'C'}=k}\\\widehat{ABH} = \widehat{A'B'H'}\end{array} \right.}$
$\color{black}{\text{Xét}}$ $\color{black}{ΔABH}$ $\color{black}{\text{và}}$ $\color{black}{ΔA'B'H'}$ $\color{black}{\text{có:}}$
$\color{black}{\left\{ \begin{array}{l}\widehat{ABH}=\widehat{A'B'H'}(cmt)\\\widehat{BHA}=\widehat{B'H'A'}=90^o\end{array} \right.}$
$\color{black}{\to ΔABH \sim ΔA'B'H'}$ $\color{black}{\text{(g.g)}}$
$\color{black}{\to \dfrac{AB}{A'B'} = \dfrac{AH}{A'H'}.}$ $\color{black}{\text{Mà}}$ $\color{black}{ \dfrac{AB}{A'B'} = k}$ $\color{black}{\to \dfrac{AH}{A'H'} = k}$
$\color{black}{\text{Suy ra điều phải chứng minh.}}$
