These are both for limit

Ok my first question is about pot equity. Could someone explain to me what it is, and inparticular how you use it in situations such as holding top pair, and if it comes into play at all when deciding what to do if you flop a middle or bottom pair?

My second question is about when you are holding middle pocket pairs.

say you have 99 in early position, you call. 3-4 players enter the pot behind you without a raise. The flop comes with only one over card. Should you bet out or check? If you check and everyone else checks to the last player, and he bets,should you call or raise as he could be bluffing? Or simply fold?

If the pot was raised in late position, how would you act then in the same two scenerios?

Up till now i have been playing these hands conservatively, i.e checking and folding to a bet, unless i am heads up with one overcard on the flop. Just wondering what others would do.

Thanks