Question

The eighteenth term of an arithmetic sequence is 243, and the ninetieth term is 516.6.
What is the 125th term?

Answer 1

\[a _{n} = a _{1} + d(n-1)\]
\[d = 516.6 - 243 = 273.6\]
\[a _{18} = 243 = a _{1} + 273.6(18-1)\]
\[\rightarrow a _{1} = -4408.2\]
\[a _{125} = -4408.2 + (273.6)(124) = 29518.2\]