|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