Question

Suatu barisan aritmatika suku ke 8=54 suku ke 15=103
a. tentukan suku ke 28
b. jumlah 20 suku pertama

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Answer 1

Suku ke 15-8 = 7
103-54 = 49
49 : 7 = 7
Maka selisih setiap suku adalah 7.
U8 = 54
U9 = (U8+7) = 54+7 = 61
U10 =(U9+7) = 61+7 = 68
U11 =(U10+7) =68+7 = 75
U12 =(U11+7) =75+7 = 82
U13 =(U12+7) =82+7 = 89
U14 =(U13+7) =89+7 = 96
U15 = 96+7 = 103
U16 = 103+7 =110
U17 = 110+7 = 117
U18 = 117+7 = 124
U19 = 124+7 = 131
U20 = 131+7 = 138
a. Maka U28 = 28-15
= 13
U28 = 13×7 = 91
U28 = 103 + 91 = 194
b. 20 suku pertama (yg diatas) dibutuhkan 7 suku lagi maka
U7 = (U8-7) = 54-7 = 47
U6 = 47-7 = 40
U5 =40-7 = 33
U4 =33-7 = 26
U3 =26-7 = 19
U2 =19-7 = 12
U1 =12-7 = 5

a. Suku ke 28 = 194
b. 5 + 12+19+26+33+40+47+54 +61+68+75+82+89+96+103+ 110+117+124+131+138 = 1430.

Tolong di teliti lagi y...
maaf kalau salah... ; )

Answer 2

B= U15-U8 = 103-54 = 49 = 7
       15-8        7            7

U8= a+(8-1) 7
54= a+ 7(7)
54= a+ 49
54-49= a
5=a

A) U28= 5+(28-1)7
             5+27(7)
             5+189
             194
B) S20= 20 {2(5)+ (20-1)7}
               2
         =10{10+133}
         =10(143)
          =1430

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