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

U5= 37
U8 = 19

U5= a + 4b
a +4b = 37

U8 = a + 7b
a + 7b = 19

a + 7b = 19
a + 4b = 37 (-)
------------------
3b = -18
b = -6

a + 4b = 37
a + 4(-6) = 37
a - 24 = 37
a = 37 + 24
a = 61

Sn = n/2(2a+(n-1)b)
S60 = 60/2 (2(61) + 59(-6))
= 30(122 - 354)
= 30(-232)
= -6 960

U₅ = a + 4b = 37
U₈ = a + 7b = 19

dieliminasi, sehingga :
-3b = 18
b = -6

a + 4b = 37
a + 4(-6)  = 37
a -24 = 37
a = 61

Sn = n/2 (2a + (n-1)b)
S₆₀ = 60/2 (2.61 + 59.(-6))
S₆₀ = 60/2 (122+ (-354))
S₆₀ = 60/2 (-232)
S₆₀ = -6960