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.}`