条件不够:
(a-2b)⊥b,即:(a-2b)·b=a·b-2|b|^2=0
即:a·b=2|b|^2
故:cos=a·b/(|a|*|b|)=2|b|^2/(|a|*|b|)
=2|b|/|a|
需要满足:|a|≥2|b|
|a|=4|b|时,cos=1/2,即:=π/3
条件不够:
(a-2b)⊥b,即:(a-2b)·b=a·b-2|b|^2=0
即:a·b=2|b|^2
故:cos=a·b/(|a|*|b|)=2|b|^2/(|a|*|b|)
=2|b|/|a|
需要满足:|a|≥2|b|
|a|=4|b|时,cos=1/2,即:=π/3