Mohon dibantu.

Bentuk umum persamaan kuadrat :
px^2 + qx + r = 0
2x^2 + x + a = 0

p = 2
q = 1
r = a

penjumlahan dan perkalian akar-akar :
α + β = -q/p = -1/2
αβ = r/p = a/2

barisan geometri :
U1 = α = a
U2 = β = Ut
U3 = 1/2 αβ = Un

rumus Ut :
Ut^2 = a × Un
β^2 = α × 1/2 αβ
β^2 = 1/2 α^2 β
β^2/β = (α^2)/2
β = (α^2)/2

masukkan β ke α + β = -1/2
α + β = -1/2
[α + (α^2)/2 = -1/2] × 2
2α + α^2 = -1
α^2 + 2α + 1 = 0
(α + 1)(α + 1) = 0
α = -1

masukkan nilai α ke β = (α^2)/2
β = (α^2)/2
= ((-1)^2)/2
= 1/2

jadi,
U1 = α = -1
U2 = β = 1/2

r = U2/U1 = 1/2/-1 = -1/2

U4 = U1 × r^(4 – 1)
= -1 × (-1/2)^3
= -1 × -1/8
= 1/8

^ = pangkat

maaf kalau salah
semoga membantu