30. 已知向量a→=(0,1,1),b→=(2,1,0)\overrightarrow{a}=(0,1,1),\overrightarrow{b}=(2,1,0)a=(0,1,1),b=(2,1,0),则a→+b→\overrightarrow{a}+\overrightarrow{b}a+b与b→−2a→\overrightarrow{b}-2\overrightarrow{a}b−2a的夹角为((( )))