# Vector ( i-j = k) ?

## Homework Statement

I cant understand why the vector MA finally become (i+j+k) ?? is it beacuse of (i-j ) becomes k ??

## The Attempt at a Solution

#### Attachments

• kgk.png
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ShayanJ
Gold Member
That's wrong!
$(i-j)\times (i+j)=\underbrace{i\times i}_0+i \times j- j\times i-\underbrace{ j \times j}_0=i\times j+i\times j=2 i \times j=2 k$

desmond iking
haruspex
Homework Helper
Gold Member
That's wrong!
$(i-j)\times (i+j)=\underbrace{i\times i}_0+i \times j- j\times i-\underbrace{ j \times j}_0=i\times j+i\times j=2 i \times j=2 k$
It's wrong before that, even.
##\vec P = P(\vec j -\vec k)/\sqrt 2##
Alternatively, by calculating the distance from A to the line of P the moment is seen to have magnitude ##aP\sqrt {\frac 32}##. I believe the correct answer is exactly half that shown.

theodoros.mihos and desmond iking
-j X i=k
That k and the other normal one from i x j=k, add up to 2k.