Suatu barisan aritmetika diketahui U4 = 11 dan U7 = 20. Suku pertama barisan tersebut adalah​

Penjelasan dengan langkah-langkah:

U4 = 11

U7 = 20

Sn ……. ?

U4 = a + 3b

11 = a + 3b

U7 = a + 6b

20 = a + 6b

20 = a + 6b

11 = a + 3b

_________–

9 = 3b

3 = b

b = 3

U4 = a + 3b

11 = a + 3(3)

11 = a + 9

11 – 9 = a

2 = a

a = 2

Sn = n/2 (2a + (n–1)b)

= n/2 (2(2) + (n–1)3)

= n/2 (4 + 3n – 3)

= n/2 (1 + 3n)