Diketahui Un adalah suku ke-n suatu deret aritmetika. Jika U1+U2+U3= -8 dan U3+U4+U5= 16 Berapa jumlah 5 suku pertama deret tersebut

A + a + b + a + 2b = -8
3a+3b = -8
a + 2b + a + 3b + a + 4b = 16
3a + 9b = 16

3a + 3b = -8
3a + 9b = 16
-6b = -24
b = 4

3a + 12 = -8
a = -20/3

S5 = 5/2 (-40/3 + 16)
S5 = 5/2 (-40/3 + 48/3)
S5 = 5/2 x 8/3
S5 = 40/6
S5 = 20/3

~Jawab~

U1 + U2 + U3 = -8
a + a + b + a + 2b = -8
3a + 3b = -8 … pers (1)

U3 + U4 + U5 = 16
a + 2b + a + 3b + a + 4b = 16
3a + 9b = 16 … pers (2)

Eliminasi

3a + 3b = -8
3a + 9b = 16
__________(-)
6a = -24
b = 4

Substitusi b

3a + 9b = 16
3a + 36 = 16
3a = -20
a = -20 /3

Sn = n/2 [2a + (n – 1) b]
S5 = 5/2 [2( -20/3 ) + 4(4)]
S5 = 5/2 [-40/3 + 16]
S5 = 5/2 ( 8/3 )
S5 = 20/3