tentukan jumlah 20 suku pertama deret matematika berikut pake caranya plissss

1.
a = U1 = 9
b = U2 - U1 = 2

S20 = n/2 [2a + 19b]
S20 = 20/2 [2(9) + 19(2)]
S20 = 10 [18 + 38]
S20 = 10 (56)
S20 = 560

2.
a = U1 = 1000
b = U2 - U1 = -4

S20 = 20/2 [2a + 19b]
S20 = 10 [2(1000) + 19(-4)]
S20 = 10 [2000 - 76]
S20 = 10 (1924)
S20 = 19240

Semoga Membantu

Mapel : Matematika
Kelas : IX SMP
Bab : Barisan dan deret

Pembahasan :
9 + 11 + 13 + 15 + ...

Sn = n/2(2a + (n - 1)b)
S20 = 10(2.9 + 19.2)
S20 = 10(18 + 38)
S20 = 10(56)
S20 = 560 —» Jumlah 20 suku pertama

————————————————
1000 + 996 + 992 + ...

Sn = n/2(2a + (n - 1)b)
S20 = 10(2.1000 + 19.(-4))
S20 = 10(2000 - 76)
S20 = 10(1924)
S20 = 19240 —» Jumlah 20 suku pertama