# Dot product and cross product of vectors

#### warfreak131

1. Homework Statement

This isn't so much a problem of calculation, so much as it is I need to know if i did it right, you'll see what I mean.

vectors v=(2,0,2), u=(-1,1,0), w=(0,-1,1)
Computer the quantities that make sense (a period denotes the dot product, x denotes cross product):

(v.u)w

(v x u) x w + 1

(v x u) + (v.u)

(v x u) . w + (v x w) . u + 1

3. The Attempt at a Solution

I said that the first one is the form of scalar * vector which equals a vector

the second is the product two cross vectors, which is a vector, plus a scalar, which does not make sense.

The third has the form of a vector plus a scalar, which doesnt make sense

and the fourth:

(v x u) . w
is a vector cross vector, which is a vector, dotted with another which is a scalar

(v x w) . u + 1
is a vector cross vector, which is a vector, dotted with a vector, which is a scalar, plus 1, which is a scalar, so youre adding three scalars, which makes sense

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#### magwas

I did not see any problem here.

cool, thanks

#### warfreak131

wait, do you mean that you dont see what the "question" is, or you dont see a problem with my answer.

the question asks to evaluate the ones that make sense. i evaluated the first and last ones on the test because i thought that those were the ones that make sense. i am asking if those are indeed the ones that can be evaluated.

#### magwas

I see the question, but don't see any error on your part.
It is easy to check this kind of things using a math package. My favourite is simpy.
You define v in simpy this way:
v=Matrix([2,0,2])
You express (v x u) . w this way:
(v.cross(u)).dot(w)
Now you got the idea.

Ahh a minor error in your part: in the last one you add two scalars, not three.

#### warfreak131

well (v x u) . w is a scalar

(v x w) . u is a scalar

and 1 is a scalar

but thanks tho

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