Question

Please help out
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

$a_1=-5;\ a_2=20;\ a_3=-80\\\\a_n=a_1r^{n-1}\\\\r=a_2:a_1\to r=20:(-5)=-4\\\\Answer:\boxed{a_n=-5\cdot(-4)^{n-1}}\to\fbox{B.}$