Why operation * not defined in Q

1. Jan 24, 2013

pakkanen

1. The problem statement, all variables and given/known data

Show that l/m * k/n = (l+k)/(m2+n2) can not be defined as an operation in Q when l,k € Z and m, n € Z\{0}

I do not know what is the issue here? Should I know something about Q that this not fulfilled by the operation *?

2. Jan 24, 2013

jbunniii

Hint: What happens if you negate $l$ and $m$?

3. Jan 24, 2013

pakkanen

Ok.. So the same operation can produce two different results??

So that l/m * k/n = (l+k)/(m2+n2) ≠ (-l+k)/((-m)2+n2) = -l/-m * k/n = l/m * k/n

4. Jan 24, 2013

jbunniii

That's right, so the operation is not well defined.

5. Jan 24, 2013

pakkanen

Thank you very much jbunniii! Helped me a lot. I think we'll meet again.