# subset help

• Sep 23rd 2009, 09:07 PM
absvalue
subset help
I need to prove that A is a subset of B if and only if A - B = the empty set.

I'm a little lost at how to do this... could someone point me in the right direction?
• Sep 23rd 2009, 09:22 PM
That is, WLOG, that A - B = ${x_i}$, some element in A.
Then A = B + ${x_i}$, but this contradicts the definition of a subset.