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.
