若向量a=(1,1),b=(2,5),c=(3,x)满足条件(8a-b)•c=30,则x=(  ) A.6 B.5 C.4 D.3

问题描述:

若向量

a
=(1,1),
b
=(2,5),
c
=(3,x)满足条件(8
a
-
b
)•
c
=30,则x=(  )
A. 6
B. 5
C. 4
D. 3

∵向量

a
=(1,1),
b
=(2,5),
8
a
b
=(8,8)−(2,5)=(6,3)

(8
a
b
)•
c
=6×3+3x=30

∴x=4.
故选C.