Ta có: \(\dfrac{AB}{AC}=\dfrac{1}{3}\)
\(\Leftrightarrow AC=3\cdot AB\)
\(\Leftrightarrow AB\cdot AC=3\cdot AB^2\)
\(\Leftrightarrow AH\cdot BC=3\cdot AB^2\)
\(\Leftrightarrow\dfrac{AH\cdot BC}{BC^2}=3\cdot\dfrac{AB^2}{BC^2}\)
\(\Leftrightarrow\dfrac{AH}{BC}=\dfrac{3AB^2}{BC^2}\)
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