Cho \(\Delta ABC\) có: \(A\left( {1;\,\,1} \right),\,\,\,B\left( {2;\,\,4} \right),\,\,C\left( {5;\,\,5} \right).\)
Ta có: \(\left\{ \begin{array}{l}\overrightarrow {AB} = \left( {1;\,\,3} \right)\\\overrightarrow {AC} = \left( {4;\,\,4} \right)\\\overrightarrow {BC} = \left( {3;\,\,1} \right)\end{array} \right. \Rightarrow \left\{ \begin{array}{l}AB = \sqrt {10} \\AC = 4\sqrt 2 \\BC = \sqrt {10} \end{array} \right..\)
\( \Rightarrow AB = BC = \sqrt {10} \Rightarrow \Delta ABC\) cân tại \(B.\)
