Dik: u2 = 6

#aritmatika
Un = a + (n – 1) b

* U2 = a + (2 – 1) b
6 = a + b

* U5 = a + (5 – 1) b
-48 = a + 4b

dieliminasi
a + b = 6
a + 4b = -48
(dikurangi)
0 – 3b = 54
-3b = 54
b = -18

a + b = 6
a + (-18) = 6
a = 6 + 18 = 24

jadi suku pertamanya adalah 24

#kalo geometri,

Un = ar^(n-1)

* U2 = ar^(2-1)
6 = ar
a = 6/r

* U5 = ar^(5-1)
-48 = a(r^4)
-48 = (6/r)(r^4)
-48 = 6(r^3)
r^3 = -8
r = -2

a = 6/r = 6/(-2) = -3

jadi suku pertamanya adalah -3