a|=√（4+m^2),|b|=√（64+m^2),a·b=2m+8m=10m,a·b=|a|*|b|cos180° =-|a||b| =-√[（4+m^2)(64+m^2)],10m=-√[（4+m^2)(64+m^2)],.(1) m^4-32m^2+256=0,(m^2-16)^2=0,m^2-16=0,m=-4,或m=4,由（1）式可知m