Question

Pada suatu barisan aritmatika, suku ke 5 = 37 dan suku ke 8 = 19. jumlah 60 suku pertama barisan tersebut adalah?

Answer 1

Cari beda
37 = a + 4b
19 = a + 7b  -
18 = -3b
-18 = 3b
-18 : 3 = b
-6 = b

cari suku awal
U5 = a + 4b
37 = a + 4b
37 = a + 4(-6)
37 = a - 24
37 + 24 = a
61 = a

S60 = 60/2 [(2 x 61) + (60-1)-6]
S60 = 30 [122 + (59 x -6)]
S60 = 30 [122 + -354]
S60 = 30 (-232) = -6960