Use the terms to write an explicit equation.

Question

a1 = 20; an+1 = ½ · an

Answer 1

Since r = a_n+1 / a_n, we have
a_n = a_1 * (a_n+1/a_n)^(n-1) = 20 * (1/2)^(n+1) = 40/2^n