# Thread: Need help with some of the vector question, especially part c, and e-h

1. ## Need help with some of the vector question, especially part c, and e-h

Suppose a point A has co-ordinates (−1, 3, 5), a point B has co-ordinates
(0, −3, 2), and another point C has co-ordinates (−5, −5, −5).
(a) Find −→BA and −−→CB.
BA(1, 0, -7)
CB(5, 8, 3)
(b) Find −→BA − 3
−−→CB.
(16, 24, 16)
(c) Use the result of part (b) to write −→BA − 3
−−→CB as a multiple of the vector (8, 0, 9).??? DON'T GET THIS PART

(d) Express −−→CB in terms of the standard unit vectors i, j and k.
-5i-8j-3k
(e) Use the result of part (a) to ﬁnd −→AC.
(f) Verify the triangle inequality: ||−→AB +
−−→BC|| ≤ ||−→AB|| + ||−−→BC||.
(g) Find a unit vector pointing in the same direction as −→AB.
(h) How far is the point C from the origin?

2. ## Re: Need help with some of the vector question, especially part c, and e-h

Originally Posted by Fearsword
Suppose a point A has co-ordinates (−1, 3, 5), a point B has co-ordinates
(0, −3, 2), and another point C has co-ordinates (−5, −5, −5).
(a) Find −→BA and −−→CB.
BA(1, 0, -7)
No, this is wrong. You seem to have gotten confused as to exactly what you subtracting from what.

CB(5, 8, 3)
Again wrong. Perhaps it would help to write out each calculation separately.

(b) Find −→BA − 3
−−→CB.
The notation is awkward- especially with the line break. I assume you mean $\displaystyle \vec{BA}- 3\vec{CB}$

(16, 24, 16)
This is wrong perhaps because you got $\displaystyle \vec{BC}$ wrong.

(c) Use the result of part (b) to write −→BA − 3
−−→CB as a multiple of the vector (8, 0, 9).??? DON'T GET THIS PART

(d) Express −−→CB in terms of the standard unit vectors i, j and k.
-5i-8j-3k
(e) Use the result of part (a) to ﬁnd −→AC.
(f) Verify the triangle inequality: ||−→AB +
−−→BC|| ≤ ||−→AB|| + ||−−→BC||.
(g) Find a unit vector pointing in the same direction as −→AB.
(h) How far is the point C from the origin?
Once you had the first part of the problem wrong, there was no chance of getting these right. If you are not certain, post exactly how you found $\displaystyle \vec{BA}$ and $\displaystyle \vec{BC}$. Perhaps we can point out your error.