Bantu sya untuk menjawab prtanyaan

U5 = 7
U9 = -9

Ditanya jumlah 20 suku pertama

Jawaban :
==========

➡Tentukan beda barisan

U9 - U5 = -9 - 7
9b - 5b = -16
4b = -16
b = -16/4
b = -4

➡Tentukan nilai a barisan

U5 = 7

a + (5-1)b = 7
a + 4(-4) = 7
a - 16 = 7
a = 7 + 16
a = 23

Maka, jumlah 20 suku pertama :

S20 = n/2 (2a + (n-1)b)

S20 = 20/2 (2x23 + (20-1)(-4))

S20 = 10 (46 - 76)

S20 = 10 x (-30)

S20 = -300


Jawaban E ✔