How do you solve this? The host's doorbell rang 20 times. The first time the doorbell rang, only one guest arrived. Each time the doorbell rang after that, two more guests arrived than had arrived on the previous ring. How many guests arrived in all at the party?

Answer 1

This is an arithmetic sequence, where your terms are
1, 3, 5, 7, ...
a = 1, d = 2

you want
sum(20)
= ..... , sub into your formula for sum(n) of an AS