Bilangan K+1, K-1, K-5 membentuk barisan geometri. Nilai k berapa?

U1 = a = K + 1, U2 = K - 1, U3 = K - 5
a = K + 1
r = U2/U1
Un = ar^(n-1)
U3 = ar²
K - 5 = (K + 1) ((K - 1)/(K + 1))²
K - 5 = (K + 1) ((K² - 2K + 1)/(K + 1)(K + 1))
K - 5 = (K² - 2K + 1)/(K + 1)
(K - 5)(K + 1) = (K² - 2K + 1)
K² - 4K - 5 = K² - 2K + 1
-4K + 2K = 1 + 5
-2K = 6
K = 6/(-2)
K = -3

semoga membantu ^^

nt: yg d gambar sama yg d ketik sama ya :D