What is ana_nan if a1=7a_1 = 7a1=7 and r=10r = 10r=10?
7⋅10n7 \cdot 10^n7⋅10n
7⋅10n−17 \cdot 10^{n-1}7⋅10n−1
70n−170^{n-1}70n−1
10⋅7n−110 \cdot 7^{n-1}10⋅7n−1