Under which of the following conditions will the points A,B,C with position vectors vec(a) , vec(b) and vec(c) respectively be collinear? (a) vec(c) −vec(a) =2(vec(b) −vec(a)) (b) |vec(c) −vec(a)|=2|vec(b) −vec(a)| (c) vec(a) =2(vec(b)+vec(c)) (d) 2 vec(a) +vec(b) =vec(c) (e) 3vec(a) −2(vec(b) +vec(c))=vec(0)

sittesf

sittesf

Open question

2022-08-16

Under which of the following conditions will the points A,B,C with position vectors a , b and c respectively be collinear?
(a) c a = 2 ( b a )
(b) | c a | = 2 | b a |
(c) a = 2 ( b + c )
(d) 2 a + b = c
(e) 3 a 2 ( b + c ) = 0
My Try
Since (a) yields A C = λ A B , it is definitely collinear.
Furthermore I know that magnitude does not affect collinearity. I'm stuck determining (c),(d),(e).

Answer & Explanation

Jewel Brooks

Jewel Brooks

Beginner2022-08-17Added 15 answers

Note that the collinearity A C = λ A B is equivalent to the vector equation
c = λ b + ( 1 λ ) a
where λ is non-zero. Then, verify that (c),(d),(e) do not satisfy the equation above.

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