Question

Tam giác ABC có AB=AC=a và \(\widehat{BAC}\) = 120 .Tính \(\left|\overrightarrow{AB}+\overrightarrow{AC}\right|\)

Answer 1

\(x=\left|\overrightarrow{AB}+\overrightarrow{AC}\right|\Rightarrow x^2=AB^2+AC^2+2\overrightarrow{AB}.\overrightarrow{AC}\)

\(x^2=AB^2+AC^2+2AB.AC.cos120^0=a^2\)

\(\Rightarrow x=a\)