2a+b=(3,3)a-b=(0,3)作出图形可知夹角为45°另：cos<2a+b,a-b>=9/(3√2*3)=√2/245°
2a+b=(3,3) a-b=(0,3)cos〈2a+b,a-b〉=3²/√3²+3²*3=√2/2夹角为45°
