12. 在ΔABC\Delta ABCΔABC中,DDD为BCBCBC的中点,EB→=2AE→\overrightarrow{EB}=2\overrightarrow{AE}EB=2AE,AF→=2FC→\overrightarrow{AF}=2\overrightarrow{FC}AF=2FC,EFEFEF与ADADAD交于GGG,AG→=λAD→\overrightarrow{AG}=\lambda \overrightarrow{AD}AG=λAD,则λ=(\lambda =(λ=( )))