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a1 = 81; a5 = 16; n = 5
16 = 81*[r^(5-1)]
16 = 81*[r^4]
r^4 = 16/81
16 = 2^4
81 = 3^4
r^4 = (2^4/3^4)
r = 2/3
Sn = [(an*r - a1)/(r - 1)]
Sn = S5
S5 = [(16*(2/3) - 81)/((2/3) - 1)]
S5 = [(32/3 - 81)/(-1/3)]
S5 = [(-211/3)/(-1/3)] = 211
S5 = 211
Rta: r = 2/3; S5 = 211