因为a⊥b,故a•b=0.因为a+2b与a-2b的夹角为120°,故cos（a+2b,a-2b）=cos120=-1/2（a+2b）•（a-2b）/（｜a+2b||a-2b|）=-1/2(a^2-4b^2)/[根号下(a^2+4a•b+4b^2)•根号下(a^2-4a•b+4b^2)...