\(\left|\overrightarrow{AB}\right|=\sqrt{\left(x_B-x_A\right)^2+\left(y_B-y_A\right)^2}=\sqrt{\left(3+1\right)^2+\left(1+1\right)^2}=2\sqrt{5}\)
\(\left|\overrightarrow{BC}\right|=\sqrt{\left(x_C-x_B\right)^2+\left(y_C-y_B\right)^2}=\sqrt{\left(6-3\right)^2+\left(0-1\right)^2}=\sqrt{10}\)
\(\left|\overrightarrow{AC}\right|=\sqrt{\left(x_C-x_A\right)^2+\left(y_C-y_A\right)^2}=\sqrt{\left(6+1\right)^2+\left(0+1\right)^2}=5\sqrt{2}\)
Áp dụng định lý hàm số cos:
\(BC^2=AB^2+AC^2-2.AB.AC.\cos\widehat{BAC}\)
\(\Leftrightarrow10=20+50-2.2\sqrt{5}.5\sqrt{2}.\cos\widehat{BAC}\)
P/s: sao ko ra được nhỉ?
