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an = r (n-1) + a1
a1 + r+a1 + 2r+a1 + ..+ 14r+a1 = 0
15a1 + (1+2+3+..14)r = 0
15 a1 + 14*15/2 r = 0
15 a1 + 7*15 r = 0
15 (a1 + 7*r) = 0
15 * a8 = 0
a8 = 0
Odp: a8 = 0