a=（m＋1）i-3j，b=i＋（m-1）ja+b=(m＋2）i ＋（m-4）ja-b=mi ＋（-m-2）j(a＋b)·（a-b）=m^2+2m-m^2+2m+8 =4m+8 =0∴m=-2
(a＋b)·（a-b）=0... ...m=-2
