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1.Ta có:$\overrightarrow {AC'}=\overrightarrow {AA'}+\overrightarrow {AC}=\overrightarrow {AA'}+\overrightarrow {AB}+\overrightarrow {AD}=\overrightarrow {a}+\overrightarrow {b}+\overrightarrow {c}$$\overrightarrow {AG}=\frac{1}{3}(\overrightarrow {AA'}+\overrightarrow {AB}+\overrightarrow {AD})=\frac{1}{3}(\overrightarrow {a}+\overrightarrow {b}+\overrightarrow {c})$$\overrightarrow {AG'}=\frac{1}{3}(\overrightarrow {AC}+\overrightarrow {AB'}+\overrightarrow {AD'})=\frac{1}{3}(\overrightarrow {AB}+\overrightarrow {AD}+\overrightarrow {AA'}+\overrightarrow {AB}+\overrightarrow {AA'}+\overrightarrow {AD})=\frac{2}{3}(\overrightarrow {a}+\overrightarrow {b}+\overrightarrow {c})$

1.Ta có:$\overrightarrow {AC'}=\overrightarrow {AA'}+\overrightarrow {AC}=\overrightarrow {AA'}+\overrightarrow {AB}+\overrightarrow {AD}=\overrightarrow {a}+\overrightarrow {b}+\overrightarrow {c}$$\overrightarrow {AG}=\frac{1}{3}(\overrightarrow {AA'}+\overrightarrow {AB}+\overrightarrow {AD})=\frac{1}{3}(\overrightarrow {a}+\overrightarrow {b}+\overrightarrow {c})$$\overrightarrow {AG'}=\frac{1}{3}(\overrightarrow {AC}+\overrightarrow {AB'}+\overrightarrow {AD'})=\frac{1}{3}(\overrightarrow {AB}+\overrightarrow {AD}+\overrightarrow {AA'}+\overrightarrow {AB}+\overrightarrow {AA'}+\overrightarrow {AD})=\frac{2}{3}(\overrightarrow {a}+\overrightarrow {b}+\overrightarrow {c})$

1.Ta có:$\overrightarrow {AC'}=\overrightarrow {AA'}+\overrightarrow {AC}=\overrightarrow {AA'}+\overrightarrow {AB}+\overrightarrow {AD}=\overrightarrow {a}+\overrightarrow {b}+\overrightarrow {c}$$\overrightarrow {AG}=\frac{1}{3}(\overrightarrow {AA'}+\overrightarrow {AB}+\overrightarrow {AD})=\frac{1}{3}(\overrightarrow {a}+\overrightarrow {b}+\overrightarrow {c})$$\overrightarrow {AG'}=\frac{1}{3}(\overrightarrow {AC}+\overrightarrow {AB'}+\overrightarrow {AD'})=\frac{1}{3}(\overrightarrow {AB}+\overrightarrow {AD}+\overrightarrow {AA'}+\overrightarrow {AB}+\overrightarrow {AA'}+\overrightarrow {AD})=\frac{2}{3}(\overrightarrow {a}+\overrightarrow {b}+\overrightarrow {c})$

1.Ta có:$\overrightarrow {AC'}=\overrightarrow {AA'}+\overrightarrow {AC}=\overrightarrow {AA'}+\overrightarrow {AB}+\overrightarrow {AD}=\overrightarrow {a}+\overrightarrow {b}+\overrightarrow {c}$$\overrightarrow {AG}=\frac{1}{3}(\overrightarrow {AA'}+\overrightarrow {AB}+\overrightarrow {AD})=\frac{1}{3}(\overrightarrow {a}+\overrightarrow {b}+\overrightarrow {c})$$\overrightarrow {AG'}=\frac{1}{3}(\overrightarrow {AC}+\overrightarrow {AB'}+\overrightarrow {AD'})=\frac{1}{3}(\overrightarrow {AB}+\overrightarrow {AD}+\overrightarrow {AA'}+\overrightarrow {AB}+\overrightarrow {AA'}+\overrightarrow {AD})=\frac{2}{3}(\overrightarrow {a}+\overrightarrow {b}+\overrightarrow {c})$