Đặt \(T=\left|\overrightarrow{a}+\overrightarrow{b}\right|\Rightarrow T^2=\overrightarrow{a}^2+\overrightarrow{b}^2+2\overrightarrow{a}.\overrightarrow{b}\)
\(=2^2+4^2+2.\left|\overrightarrow{a}\right|.\left|\overrightarrow{b}\right|.cos\left(\overrightarrow{a};\overrightarrow{b}\right)\)
\(=20+2.2.4.cos120^0=12\)
\(\Rightarrow T=\sqrt{12}=2\sqrt{3}\)
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