# Vector ( i-j = k) ?

1. Jan 10, 2015

### desmond iking

1. The problem statement, all variables and given/known data
I cant understand why the vector MA finally become (i+j+k) ?? is it beacuse of (i-j ) becomes k ??

2. Relevant equations

3. The attempt at a solution

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2. Jan 10, 2015

### ShayanJ

That's wrong!
$(i-j)\times (i+j)=\underbrace{i\times i}_0+i \times j- j\times i-\underbrace{ j \times j}_0=i\times j+i\times j=2 i \times j=2 k$

3. Jan 10, 2015

### haruspex

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.

4. May 18, 2015

### Don Bari

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