Soal deret dan barisan aritmatika 2, 3/2, 1, 1/2, ... , -5

2, 3/2, 1, 1/2, ...., -5
a = 2
b = 3/2 - 2 = 3/2 - 4/2 = -1/2

Un = -5
a + (n - 1)b = -5
2 + (n - 1)(-1/2) = -5
(n - 1)(-1/2) = - 5 - 2
(n - 1)(-1/2) = -7
(n - 1) = -7 . (-2/1)
n - 1 = 14
n = 15
Jadi banyaknya suku ada 15