tentukan banyak suku disetiap barisan berikut . a. 135,130,125, . . . ,35 b. 3,1,-1, . . . ,-61

Deret aritmatika
a, a+b, a+2b,.......a+(n-1)b

a = 135
b = 130 - 135 = -5
Un = a + (n-1)b
35 = 135 + (n-1)(-5)
35 = 135 - 5n + 5
35 = 140 - 5n
5n = 105
n = 21

a = 3
b = -2
Un = a + (n-1)b
-61 = 3 + (n-1)(-2)
-61 = 3 - 2n + 2
-61 = 1 - 2n
2n = 62
n = 31