Tentukan jumlah 20 suku pertama bila ketahui U3=12 dan U7=32

Mapel : Matematika

Kelas : IX

Bab : Barisan dan Deret
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U3 = 12
U7 = 32

Ditanya :
↪S20 =...?

Jawaban :

→Tentukan beda barisan

U7 - U3 = 32 - 12

7b - 3b = 20

4b = 20

b = 20/4

b = 5 ✔


→Tentukan nilai a

U3 = 12

a + (3-1)b = 12

a + 2(5) = 12

a + 10 = 12

a = 12 - 10

a = 2 ✔



Maka, nilai S20

S20 = n/2 (2a + (n-1)b)

S20 = 20/2 (2 x 2 (20 - 1)(5))

S20 = 10 (4 + 19(5)

S20 = 10 ( 4 + 95)

S20 = 10 x 99

S20 = 990✔