U4 = 8 U8 = 16 tentukan : jumlah suku ke 8

U4 = a + 3b = 8
U8 = a + 7b = 16
------------------------- -
                -4b = -8
                   b = 2

a + 3(2) = 8
a + 6 = 8
a = 2

S8 = 8/2(2+16)
      = 4 x 18
      = 72

Jawab:
U4: a+3b = 8
U8: a+7b = 16 * dieliminasi
4b = 8
=> b = 2
subtitusi ke salah satunya,,
a+3b=8
a+3(2)=8
a+6=8
a=8-6 => a=2

S8 = n/2(2a+(n-1)b)
=> 8/2(2(2)+(7)2)
=> 4(4+14)
=> 4(18)
=> 72

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