Given the following?
a1 = 28
a2 = 21
a3 = 14
a4 = 7
We are asked to find the formula for the nth term in above arthmetic sequence.
Generally the nth term of an arithmetic sequence is:
an = a1 + (n - 1)d
Where:
a1 is the first term
d is the common difference.
Lets find the common difference, d:
d = a2 - a1 = a3 - a2
d = 21 - 28 = 14 - 21
d = -7 = -7
d = -7
So,
an = a1 + (n - 1)d
an = 28 + (n - 1)-7
an = 28 -7n + 7
an = -7n + 28
