tentukan s8 dari barisan aritmatika jika s2 =-2 dan s7 =8

S2 = a + b = -2
S7 = a + 6b = 8
---------------------- --
-5b = -10
b. = 2

S2 = a + b
-2 = a + 2
a = -4

S8 = a + 7b = -4 + 7(2) = -4 + 14 = 10

Diketahui:
S2 = -2
U1+U2 = -2
a+(a+b) = -2
2a+b = -2 ----------->> (1)

S7 = 8
U1+U2+U3+U4+U5+U6+U7 = 8
a+(a+b)+(a+2b)+(a+3b)+(a+4b)+(a+5b)+(a+6b) = 8
7a + 21b = 8 ------->> (2)

Ditanya:
S8 = ???

Jawab:
Eliminasikan 1 dan 2 ↓↓↓
2a +     b = -2 ..................... (dikali 21)
7a + 21b =  8 ..................... (dikali 1)
_____________

42a + 21b = -42
  7a + 21b =    8
_____________ -

35a = -50
a = -50/35
a = -10/7

Subtitusikan "a" ke 1 ↓↓↓
2a + b = -2
2(-10/7) + b = -2
-20/7 + b = -2
b = -2 + 20/7
b = 6/7

Kemudian cari nilai "U8" ↓↓↓
Un = a + (n-1)b
U8 = -10/7 + (8-1)6/7
U8 = -10/7 + 6
U8 = 32/7

Maka ↓↓↓
S8 = S7 + U8
S8 = 8 + 32/7
S8 = 88/7
S8 = 12,57