# Math Help - closed under subtraction

1. ## closed under subtraction

how do i prove that for any a,b in Z, the set L sub(a,b) is closed under subtraction

2. Originally Posted by alyssams
how do i prove that for any a,b in Z, the set L sub(a,b) is closed under subtraction

It'd be a very good idea if you'd tell what in the world is Lsub(a,b)...

Tonio

3. sorry, La,b is (ax+by:x,y element of Z)

4. $(ax+by)-(ax'+by') = a(x-x')+b(y-y)'\in L_{a,b}$?