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# hmmm.... watch

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1. sorry to start another thread on this but i still don't get it!!

the question: solve y+(axy)=b (where x represents cross product) for x in terms of a and b.

what i did is as follows...

i took the cros product of each side with a:
axy+ax(axy)=axb
∴axy+axa+axy=axb
∴2axy=axb
∴y=0.5b

why is this wrong and what is the right way? I have been told i should take dot product and scalar product of both sides, but i dont see how this helps.
confused!!!
2. (Original post by realicetic)
axy+ax(axy)=axb
∴axy+axa+axy=axb
∴2axy=axb
∴y=0.5b
ax(axy) is not equal to axa + axy I dont think...
axy = n|a||y|sinØ. Since a is perpendicular to n,then axn = |a|²|n||y|k roughly(K perp to n and a(is this y?)), which is not the same as axy.
I havent done dot and cross in a while, but I suspect that you can do this:
dot a to everything, then you obtain
a.y+ a.(axy) = a.b
Since a.y will produce a perpendicular vector to a, a.(axy) = 0. (as the angle between then is now 90°, cos 90 = 0).

Hence a.y = a.b.
You can do the rest if you can go further, cause my memory just ran out

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