Question

Write the explicit formula for the geometric sequence.

a1 = -5 a2 = 20 a3 = -80

A. an = -5 • (-4)n

B. an = -5(-4)n-1

C. an = -4(-5)n-1

D. an = -5 • (4)n

Answer 1

The answer is

B. an = -5(-4)n-1

Answer 2

We are given the series of a1 = -5 a2 = 20 a3 = -80. In this case, we can see that this is a geometric sequence with a geometric ratio equal to 20/-5 = -80/ 20 or equal to 4. Hence, r is equal to 4, a1 is equal to -5.

The geometric sequence formula is an = a1*r^(n-1) = -5 * (-4)^ (n-1) where n is an integer. Answer is B.