1. The first step in proving a formula by ________ ________ is to show that the formula is true when n = 1.
2. To find the ________ differences of a sequence, subtract consecutive terms.
3. A sequence is an ________ sequence when the first differences are all the same nonzero number.
4. If the ________ differences of a sequence are all the same nonzero number, then the sequence has a perfect quadratic model.