Vector Cross Product: Calculating -i x i = 0?

[Ry122]

What is the cross product of -i x i? Is it negative 1 or is still just 0?

 

[Defennder]

Note that for any 2 vectors a,b: -a x b = -(a x b). This reduces the problem to -(i x i).

Now what is the vector product of a vector with itself?

 

[Ry122]

well can you tell me how the projection of u on to v
where
u=-i+2j and v=i+2j is
v=(3/5)i+(6/5)j ?
The answer i got was (4/5)i + (8/5)j
i used the equation
w=v.((u.v)/(modulusv^2))

 

[D H]

i used the equation
w=v.((u.v)/(modulusv^2))

This equation for the projection is

$$\mathbf w = \mathbf v \frac {\mathbf u \cdot \mathbf v}{v^2}$$

Note well: The cross product is not involved when you compute the projection this way.

 

[Ry122]

That's the same equation that I gave. yeah i realized my mistake after posting, i should have said dot product, not cross product.