dari suatu barisan aritmetika, diketahui U10 =41 dan U5 = 21.U20 barisan tersebut adalah

Materi : Barisan dan Deret
Kelas : IX SMP

U10 = U1 + 9b = 41
U5 = U1 + 4b = 21
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========> 5b = 20
========> b= 4

U5 = U1 + 4b
21 = U1 + 4(4)
21 = U1 + 16
U1 = 21 - 16
U1 = 5

Un = U1 + ( n - 1 ) b
U20 = 5 + ( 20 - 1 ) 4
U20 = 5 + 19 . 4
U20 = 5 + 76
U20 = 81

U5 = 21

U10 = 41
U5 + 5b = 41
21 + 5b = 41
b = 20/5 = 4

U20 = U10 +10b = 41 + 10(4) = 41 + 40 = 81