Notice that the given information allows for two possible values for r, +4 and –4. The sequence
might be this:
3, 12, 48, 192,...
Or it might be this:
3, –12, 48, –192,...
The only difference in the latter is that the second, fourth, and all other even-numbered terms are
negative. You’re looking for the eleventh term, which is an odd-numbered term, so it will be the
same number in either case. For simplicity’s sake, use r = 4 in the formula:
The answer is (D).
If you don’t already at least half-know these formulas, they’re probably not worth worrying about.
But if you can remember them without too much trouble, one of them might come in handy on Test
Day.
Finally, here are a few other topics that occasionally appear in the test.