B₁=-45
b₂=15
b₃=-5
S₈-?
q=b₂/b₁=15/(-45)=-1/3
S₈=b₁(q⁸-1) =-45((-1/3)⁸-1) = - 3² * 5 * (1 - 3⁸) = 3² * 5 *(1-3⁸) = 5 * (1-3⁸) =
q-1 (-1/3) - 1 (-4/3) * 3⁸ 4 * 3⁷ 4* 3⁵
1) 45=9*5=3² * 5
2) (-1/3)⁸ - 1= 1 - 1 = 1 - 3⁸
3⁸ 3⁸
3) (-1/3) - 1 = -4
3
= 5 * (1-6561) = 5 * (-6560) = -5 * 1640 = -8200 = -33 ¹⁸¹/₂₄₃
4 * 243 4 * 243 243 243
Ответ: -33 ¹⁸¹/₂₄₃