Example of writing a rule for a geometric sequence using the formula \( a_n = a_1 r^{n-1} \), with terms and calculations shown.
A geometric sequence formula example on lined notebook paper, showing the rule \( a_n = a_1 r^{n-1} \) with the sequence 3, 15, 75, 375, 1,875, and calculation of the 9th term.