This is our base case. We have $S_1 = a$ on the left side. On the right side, we substitute $n=1$ into the formula to get $a - a \cdot r^{1}$ divided by $1-r$. Simplifying this gives us $a \cdot (1 - r)$ divided by $1-r$, which is equal to $a$. Since the left side
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