0=(a+b)(2a-b)=2|a|^2+ab-|b|^2,0=(a-2b)(2a+b)=2|a|^2-3ab-2|b|^2,3[|b|^2-2|a|^2]=3ab=2|a|^2-2|b|^2,|b|^2=(8/5)|a|^2,ab=2/5|a|^2,余弦=ab/[|a||b|]=(2/5)/(8/5)^(1/2)=-(10)^(-1/2)