Question

The nineteenth term in an arithmetic

sequence is 243, and the eleventh term is 147.
What is the value of the eighty-sixth term?

Answer 1

1047

Explanation:

a_n = a_1 + (n - 1)d

a_19 = 243

a_11 = 147

243 = a_1 + (19 - 1)d

147 = a_1 + (11 - 1)d

243 = a_1 + 18d

147 = a_1 + 10d

a_1 = 243 - 18d

a_1 = 147 - 10d

243 - 18d = 147 - 10d

-8d = -96

d = 12

a_1 = 147 - 10d

a_1 = 147 - 10(12)

a_1 = 27

The first term is 27. The common difference is 12.

a_86 = a_1 + (n - 1)d

a_86 = 27 + 85(12)

a_86 = 1047