1.Diketahui barisan: 19,14,9,4,…

Jawaban:

Barisan Dan Deret

a = 19

b = 14 – 19 = -5

.

Suku ke-n

Un = a + (n – 1) × b

Un = 19 + (n – 1) × (-5)

Un = 19 + (-5n) – (-5)

Un = 19 – 5n + 5

.

Suku ke-91

U91 = 19 – (5 × 91) + 5

U91 = 19 – 455 + 5

U91 = -436 + 5

U91 = –431

.

Suku bilangan 91

-91 = 19 – 5n + 5

-91 = 14 – 5n

-5n = 14 – (-91)

-5n = 105

n = 105/5

n = 21