1 способ:
q=-20 : 40
q=-0.5
b₁=40
b₂=-20
b₃=10
b₄=-5
b₅=2.5
b₆=-1.25
S₆=b₁+b₂+b₃+b₄+b₅+b₆
S₆=40-20+10-5+2.5-1.25
S₆=26.25
2 способ:
q=-0.5=-1/2
b₆=b₁*q⁵
b₆=40*(-1/2)⁵
b₆=-40
32
b₆=-5/4
S₆=b₆*q - b₁ =(-5/4) * (-1/2) - 40 = (5/8) - 40 = 5-320 * (-2) =
q-1 (-1/2) - 1 -3/2 8 3
= -315 * (-2) = 105 * 1 = 26.25
8 3 4 1
Ответ: 26,25