tolong dibantu teman*..sama caranya ya..

18. Barisan/Deret Aritmatika

U7 = a + 6b = 30
U3 = a + 2b = 18
______________ - (eliminasi)
4b = 12
b = 3

maka
a+2b = 18
a+2.3 = 18
a + 6 = 18
a = 12

Un = a + (n-1)b
144 = 12 + (n-1)3
144 = 12 + 3n - 3
3n = 144 - 9
3n = 135
n = 45 (D)

19. Deret Geometri Tak hingga

a(suku pertama) = 54
r(rasio) = 18/54 = 1/3

Sn = a/(1-r)
Sn = 54/(1-1/3)
Sn = 54/ 2/3
Sn = 54 × 3/2
Sn = 81 (E)

20.Deret Aritmatika

U3 + U8 = 50
(a+2b) + (a+7b) = 50
2a + 9b = 50

U25 - U18 = 28
(a + 24b) - (a + 17b) = 28
7b = 28
b = 4

masukkan ke persamaan
2a + 9b = 50
2a + 9.4 = 50
2a + 36 = 50
2a = 14
a =7

Jumlah 5 suku terakhir
U26 + U27 + U28 + U29 + U30
=(a + 25b)+ (a+26b)+(a+27b) +(a+28b)+ (a+29b)
=5a + 135b
=5.7 + 135.4
=35 + 540
= 575 (A)

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