If you notice, in the sum the terms are 2, -8, 32, -128....
we can always get the "common ratio" of a geometric sequence by dividing the "following term by the previous term", namely like in this case say -8/2, which is -4, so r = -4, and we know the first term is 2.
so, notice, the pattern will then be
we can always get the "common ratio" of a geometric sequence by dividing the "following term by the previous term", namely like in this case say -8/2, which is -4, so r = -4, and we know the first term is 2.
so, notice, the pattern will then be