diketahui suatu barisan aritmatika suku ke-2=28, suku ke -10= 52 jumlah 81 suku pertama adalah

U2 : a + b = 28
U10 : a + 9b = 52

a + b = 28
a = 28 - b

a + 9b = 52
(28 - b) + 9b = 52
28 + 8b = 52
8b = 52 - 28
8b = 24
b = 24/8
b = 3

a = 28 - b
a = 28 - 3
a = 25

S81 = 81/2 (2(25) + 80(3))
S81 = 81/2 (50 + 240)
S81 = 81/2 (290)
S81 = 11.745