Cho tam giác ABC có trọng tâm G. Khi đó:

A. $\overrightarrow {AG} = \frac{1}{2}\overrightarrow {AB} + \frac{1}{2}\overrightarrow {AC}$

B. $\overrightarrow {AG} = \frac{1}{3}\overrightarrow {AB} + \frac{1}{2}\overrightarrow {AC}$

C. $\overrightarrow {AG} = \frac{1}{3}\overrightarrow {AB} + \frac{1}{3}\overrightarrow {AC}$

D. $\overrightarrow {AG} = \frac{2}{3}\overrightarrow {AB} + \frac{2}{3}\overrightarrow {AC}$

$\begin{array}{l}\overrightarrow {GA} + \overrightarrow {GB} + \overrightarrow {GC} = \overrightarrow 0 \Leftrightarrow \overrightarrow {AG} = \overrightarrow {GB} + \overrightarrow {GC} \\ \Leftrightarrow \overrightarrow {AG} = \overrightarrow {AB} - \overrightarrow {AG} + \overrightarrow {AC} - \overrightarrow {AG} \Leftrightarrow 3\overrightarrow {AG} = \overrightarrow {AB} + \overrightarrow {AC} \Leftrightarrow \overrightarrow {AG} = \frac{1}{3}\overrightarrow {AB} + \frac{1}{3}\overrightarrow {AC} \end{array}$

Chọn C