I am assuming its ok to discuss an OG problem w/o actually reproducing the question/explanation text.

DS Q89.

The answer is D.

A. I was able to arrive at the operation. However, I am not sure how knowing the operation is alone sufficient here. Think about when k = 0, which would satisfy the eqn and when and k = 2 (say), which won't.

B. Same as above

Hey, technically it is okay to discuss problems online without reproducing them, but we don't do it here because it's just too cumbersome for everybody to have to look up all the problems. Ideally, each post helps not just the poster but everyone else viewing it, and that's hard to do if the problem itself isn't sitting there! Since we have limited time, we want to spend it on things that will help the maximum number of people.

I'll address this one for you because I've been asked it before and I know the answer already but, just an FYI to everyone that we can't do this on a regular basis. If you're in a class, ask your teacher before or after class or sign up for your office hours (go do it right now!).

Re-read the question. It asks whether the statement is true for ALL numbers k, l and m. As you just pointed out, it is NOT true for ALL numbers. This is a yes/no question and you just answered it: no. :)

This one was subtle. I didn't consider the question completely. I understand the reasoning after you pointed it out. Thanks for answering it inspite of your policies that discourage it.
I will comb through OG more thoroughly for such hidden gems :).

We're glad it helped.

Rey