U3 = 24
U6 = 36
Ditanyakan :
S15 = ...?
Jawab :
Rumus :
Un = a + ( n-1)b
Sn = n/2 ( 2a + ( n-1) b)
U3 = 24
a + 2b = 24...................(1)
U6 = 36
a + 5b = 36....................(2)
Eliminasikan persamaan (1) dan (2)
a + 2b = 24
a + 5b = 36
---------------- -
- 3b = -12
b = 4
Substitusikan nilai b ke salah satu persamaan
a + 2b = 24
a + 8 = 24
a = 16
S15 = 15 / 2 ( 2a + ( n- 1) b )
= 15/ 2 ( 2(16) + (14)4)
= 15/2 ( 32 + 56 )
= 15/2 ( 88)
= 15 x 44
= 660
Simak video pembahasannya disini yah!
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