|a|²=|b|²=|a-b|²
=|a|²+|b|²-2ab
ab=|a|²/2=|b|²/2
|a+b|²=|a|²+|b|²+2ab=3|a|²
cos(a,a+b)=a(a+b)/(|a||a+b|)=[|a|²+|a|²/2] / [|a|²√(3)=√3/2
夹角为30度
|a|²=|b|²=|a-b|²
=|a|²+|b|²-2ab
ab=|a|²/2=|b|²/2
|a+b|²=|a|²+|b|²+2ab=3|a|²
cos(a,a+b)=a(a+b)/(|a||a+b|)=[|a|²+|a|²/2] / [|a|²√(3)=√3/2
夹角为30度