Question

An environmental organization started a new online discussion group. The group had 25 members on its first day, and added 20 new members every day after that. Which recursive sequence correctly shows the number of members the environmental organization has after n days?


a
1
=
25

a
n
=
a
n
−
1
+
20


a
1
=
25

a
n
=
a
n
+
1
+
20


a
0
=
25

a
n
=
a
n
+
1
+
20


a
0
=
25

a
n
=
a
n
+
20

Answer 1

The first option:

a1 = 25
a_n = a_{n−1} + 20 for n ≥ 2

Explanation: Day 1 has 25 members, and each day you add 20 to the previous day's total.