Ta có: \(\frac{AB}{AC}=\frac{5}{12}\)
\(\Leftrightarrow AB=\frac{5\cdot AC}{12}\)
Áp dụng định lí pytago vào ΔABC vuông tại A, ta được:
\(AB^2+AC^2=BC^2\)
\(\Leftrightarrow\left(\frac{5\cdot AC}{12}\right)^2+AC^2=39^2\)
\(\Leftrightarrow AC^2\cdot\frac{25}{144}+AC^2=1521\)
\(\Leftrightarrow AC^2\left(1+\frac{25}{144}\right)=1521\)
\(\Leftrightarrow AC^2\cdot\frac{169}{144}=1521\)
\(\Leftrightarrow AC^2=1521:\frac{169}{144}=1296\)
\(\Leftrightarrow AC=\sqrt{1296}=36cm\)
Ta có: \(AB=\frac{5\cdot AC}{12}\)(cmt)
hay \(AB=\frac{5\cdot36}{12}=\frac{180}{12}=15cm\)
Vậy: AB=15cm; AC=36cm