It seems like you are asking about a geometric sequence, not an arithmetic sequence. An arithmetic sequence is a sequence of numbers that have a constant difference between consecutive terms, such as 3, 5, 7, …1. A geometric sequence is a sequence of numbers that have a constant ratio between consecutive terms, such as 2, 4, 8, …2. The formula for the nth term of an arithmetic sequence is a_n = a_1 + (n-1)d3, where a_1 is the first term, d is the common difference, and n is the term number. The formula for the nth term of a geometric sequence is a_n = a_1 r^(n-1)4, where a_1 is the first term, r is the common ratio, and n is the term number. To find BC, you need to use the formula for a geometric sequence, not an arithmetic sequence. I hope this helps you understand the difference between arithmetic and geometric sequences.