|b|=2|a| 得：b^2=4a^2
|a-b|=sqrt(3)*|a|,两边平方得：2ab=b^2-2a^2,
ab=a^2
cos[a^(a+b)]=a(a+b)/|a||a+b|=2a^2/[|a|*sqrt(a^2+b^2+2ab)]
=2a^2/[|a||a|sqrt(7)]
=2/sqrt(7)