this formula, doesn't rely on a product, relies on a "sum", or is namely an arithmetic sequence, aₙ₋₁ -3 is another way of saying, the value of the previous term minus 3, so it relies on the ordinal value of a term, so is an recursive formula, well, let's get it when n = 4.
[tex]\begin{array}{ccll} order&term&value\\ \cline{1-3} 1&a_1&8\\ 2&a_2&a_{2-1}-3\\ &&a_1-3\\ &&8-3\\ &&5\\ 3&a_3&a_{3-1}-3\\ &&a_2-3\\ &&2\\ 4&a_4&a_{4-1}-3\\ &&a_3-3\\ &&2-3\\ &&-1 \end{array}[/tex]
