Problem
Algebra \( 2> \) CC. 19 Partial sums of geometric series 9JU Find the \( 12^{\text {th }} \) partial sum of the geometric sequence \( a_{n}=3 \cdot 2^{n-1} \).
Solution
Step 1 ：\( S_{12} = \frac{a_1(1 - r^{12})}{1 - r} \)
Step 2 ：\( S_{12} = \frac{3(1 - 2^{12})}{1 - 2} \)
Step 3 ：\( S_{12} = 3(1 - 2^{12}) \)
