|a＋b|^2-|a－b|^2=4ab=0 则ab=0|a－b|^2=a^2-2ab+b^2=a^2+b^2=(2|a|)^2=4a^2 则b^2=3a^2故cosx=(a-b)(a+b)/(|a-b|*|a+b|)=-2a^2/4a^2=-1/2则x=120°即向量a＋b与向量a－b的夹角是120°