Question

Use the formula to evaluate the series 3-9+27-81+...-a8

Answer 1

In the given series, we see that the first term is 3 and the common ratio between the succeeding terms is -3. To solve for the sum of the first 8 terms, we use the equation,

                                    Sum = (a1)(1 – r^n) / (1 – r)

Substituting the known terms,

                                    Sum = (3)(1 – (-3)^8)/(1 - -3) = -4920

Thus, the answer is -4920.