已知向量a=(x1,y1,z1),b=(x2,y2,z2),若a≠b,设| a-b ︳=R,则a-b与x轴夹角的余弦值为x1-x2∕R B.x2-x1∕R C.︳x1-x2︳∕R D.±（x1-x2）∕R
问题描述：
已知向量a=(x1,y1,z1),b=(x2,y2,z2),若a≠b,设| a-b ︳=R,则a-b与x轴夹角的余弦值为
x1-x2∕R B.x2-x1∕R C.︳x1-x2︳∕R D.±（x1-x2）∕R
答
选D
答
a-b
=(x1-x2,y1-y2,z1-z2)
|a-b| = R
let t = a-b与x轴夹角的余弦值
cost = (a-b).(1,0,0) /R
= (x1-x2)/R
Ans :A