Soal Ujian Matematika IPA Kelas 12
Tahun 2018Soal Pilihan Ganda
12. Diketahui suku ke-2 dan suku ke-6 suatu deret aritmetika berturut-turut 28 dan 60. Jumlah 20 suku pertama deret tersebut ialah ...
A. 1910
B. 1920
C. 1930
D. 1940
E. 1950
Jawaban : B
U2 = 28
U6 = 60
Mencari b
U6 - U2 = ( 6 - 2 ) x b
60 - 28 = 4 b
32 = 4 b
b = 8
Mencari a
Un = a + (n-1).b
U2 = a + (2-1).8
28 = a + 8
a = 28 - 8
a = 20
S20 = 1/2 n ( 2a + (n - 1) . b)
S20 = 1/2. 20 ( 2.20 + (20 - 1).8 )
S20 = 10 ( 40 + 19.8 )
S20 = 10 ( 40 + 152 )
S20 = 10 ( 192 )
S20 = 1920
U2 = 28
U6 = 60
Mencari b
U6 - U2 = ( 6 - 2 ) x b
60 - 28 = 4 b
32 = 4 b
b = 8
Mencari a
Un = a + (n-1).b
U2 = a + (2-1).8
28 = a + 8
a = 28 - 8
a = 20
S20 = 1/2 n ( 2a + (n - 1) . b)
S20 = 1/2. 20 ( 2.20 + (20 - 1).8 )
S20 = 10 ( 40 + 19.8 )
S20 = 10 ( 40 + 152 )
S20 = 10 ( 192 )
S20 = 1920
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