cosABC=cosACB
=> $\frac{\left |n_{AB}.n_{BC} \right |}{\left | n_{AB} \right |.\left | n_{BC} \right |}=\frac{\left | n_{BC}.n_{AC} \right |}{\left | n_{BC} \right |.\left | n_{AC} \right |}$
=>$\frac{\left | 2+1 \right |}{\sqrt{2}.\sqrt{5}}=\frac{\left | 2.A-B \right |}{\sqrt{A^{2}+B^{2}}.\sqrt{5}}$