# Math Help - vectors

1. ## vectors

I was given: a=2i - 3j - k, b=2i - j and c = -3j + k

(a) Find a + b, |a + b| Which I'm sure I'm correct with)
a + b = 4i - 2j - k

|a + b| = *33 (* representing a square root)

Here I found -(a+c), assuming it would be interpreted in the direction c towards a ??
That is a + c = 2i - 2k
I found unit vector then to be: - 4/*40 (2i - 6j)

And therefore unit vector a-c: 4/*40 (-2i + 6j)

I'm confused, because if I did it a-c I'd get 0/*8

thanks for any help

2. ## Re: vectors

Originally Posted by cellae
I was given: a=2i - 3j - k, b=2i - j and c = -3j + k

(a) Find a + b, |a + b| Which I'm sure I'm correct with)
a + b = 4i - 2j - k

|a + b| = *33 (* representing a square root)

Here I found -(a+c), assuming it would be interpreted in the direction c towards a ??
That is a + c = 2i - 2k
I found unit vector then to be: - 4/*40 (2i - 6j)

And therefore unit vector a-c: 4/*40 (-2i + 6j)

I'm confused, because if I did it a-c I'd get 0/*8

thanks for any help
Are you sure your a+b is correct? a=2i-3j-k and b=2i-j gives a+b=4i-4j-k.
a-c=2i-3j-k-(-3j+k)=2i-2k
Unit vector in direction of a-c : $\frac{1}{\sqrt{2}}$(i-k)

3. ## Re: vectors

Oh cool !

I then got asked: Find a unit vector to b and to a-c:

So I got b - b.(a-c)/|a-c|

= b - 4/4 (2j - 2k)
b - (2j - 2k)

thus (2i - j) - (2j - 2k)
= 2i - 3j - 2k

(seems wrong to me :/)

4. ## Re: vectors

just figured i was wrong there.. all good