tolong pake cara ya..

Bab Barisan dan Deret
Matematika SMP Kelas VIII

Aritmatika

U10 = -24
U3 + U7 = 12

U20 = ?

U3 + U7 = 12
a + 2b + a + 6b = 12
2a + 8b = 12
a + 4b = 6 → U5

a + 9b = -24 → U10
a + 4b = 6
------------------- -
5b = -30
b = -6

U20 = U10 + 10 x b
U20 = -24 + 10 x (-6)
U20 = -24 - 60
U20 = -84