Originally Posted by

**andrew2322** Hello all, not sure if this is the right thread but i need to check some of my answers to some problems. thanks.

if u = i + 2j - k and v = j - 3k are vectors in R^3. Find:

a) a unit vector poiting in the direction of -3u:

my solution: -3 (i + 2j - k) = -3i - 6j +3k

so then the magnitude is: root (9 +36 + 9) which is root (54) so then the unit vector is 1 / root(54) * (3i + 6j - 3k)

b) u * (12v)

i used the fact that u * (12v) is equal to 12 (u * v)

i first found u * v using the dot product formula to get 5. so then 12 (5) = 70.

c) the cosine of the angle between u and v.

I used the product i found earlier (5) and used the formula cos theta = u * v / (absolute value of u * absolute value of v) and got the answer 5 / root (60)