What's the answer?

[FallentoReason]

You would have a point, if you ignore the fact that he shows the entire calculation:

(1-1)(1+1)/1-1 = 1(1-1)/1-1
(1+1) = 1

You can only get to step 2 above if you take the 1-1 in parentheses. So yes, he was dividing by zero. He just wasn't writing it down properly.

Intent was very clear though.

Fair enough. As an engineering student though, I have to be a wanker about that sort of stuff. In an exam you get marked wrong for it.