Question

Vectors question

Answer 1

OA = ( 2 1 3 ) OB = ( 0 -1 7 ) OC = ( 2 4 7 )
Prove that cos BAC = 1/3

Answer 2

have you tried finding AB,BC and AC
and then using cosine rule?

Answer 3

AB = magnitude of (OB-OA)

similarly, find BC,AC

Answer 4

No we are required to use the dot product formula @hartnn

Answer 5

and even using the cosine rule the answer isn't coming

Answer 6

ok, dot product is easier!

find vectors AB, AC only,
then
AB.AC = |AB|.|AC| cos <BAC

tried this?

Answer 7

yep

Answer 8

AB = (-2, -2, 4)
AC = (0, 3, 4)

AB.AC = -6+16 = 10

|AB| = sqrt 24
|AC| = 5

cos BAC = 10/(5* sqrt 24)

oh yeah, its not coming out to be 1/3

Answer 9

I know that's what freaking me out.

Answer 10

you got the same cos value?

Answer 11

yes

Answer 12

and the same with the cosine rule

Answer 13

then the question is incorrect :P :3

Answer 14

It's from a board exam

Answer 15

Yeah I think the question is wrong.