В₁=40
В₂=-20
В₃=10
S₆-?
q=B₂ = 40 = - 1 =-0.5
B₁ -20 2
Первый способ:
В₄=В₃*q=10 * (-¹/₂)=-5
B₅=B₄*q=-5 *(-¹/₂)=2.5
B₆=B₅*q=2.5 * (-¹/₂)=-1.25
S₆=B₁+B₂+B₃+B₄+B₅+B₆
S₆=40-20+10-5+2.5-1.25=52.5-26.25=26.25
Второй способ:
В₆=В₁*q⁵=40 * (-¹/₂)⁵=-40 =-5 =-1.25
32 4
S₆=B₆*q - B₁ =-1.25 * (-0.5) - 40 =0.625 - 40 =26.25
q-1 -0.5-1 -1.5