a) Ta có: $\overrightarrow{a}.\overrightarrow{b}=2.6+(-3).4=0$
=> $\cos (\overrightarrow{a}.\overrightarrow{b})=0$
=> $(\overrightarrow{a}.\overrightarrow{b})=90^{\circ}$
b) Ta có: $\overrightarrow{a}.\overrightarrow{b}=13$
=> $\left | \overrightarrow{a} \right |=\sqrt{13}$
$\left | \overrightarrow{b} \right |=\sqrt{26}$
=> $\cos (\overrightarrow{a}.\overrightarrow{b})=\frac{\sqrt{2}}{2}$
=> $(\overrightarrow{a}.\overrightarrow{b})=45^{\circ}$
c) Ta có: $\overrightarrow{a}.\overrightarrow{b}=-12$
=> $\left | \overrightarrow{a} \right |=\sqrt{16}$
$\left | \overrightarrow{b} \right |=2\sqrt{3}$
=> $\cos (\overrightarrow{a}.\overrightarrow{b})=\frac{-\sqrt{3}}{2}$
=> $(\overrightarrow{a}.\overrightarrow{b})=150^{\circ}$