Hello,

You have to check that for any u,v belonging to the set, u+v belongs to the set, and for any real number , belongs to the set.

So let and , where and .(a)

Does this belong to ?

Consider the case when

Let and(b)

Can the first coordinate be in the form ?

The problem here is that s and t are the same in the first and the second coordinates.

If there weren't, you could have just written

But you would have to get as your second coordinate...