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U1 +11b = 36
-------------------------- -
-5b = -15
b = -15/-5 = 3
U1+6(3) = 21
U1 + 18 = 21
U1 = 21-18
U1 = 3
Sn= n/2(2u1 + (n-1)b))
S18 = 18/2 (2(3) + (18-1) 3))
= 9 (6+ (17x3))
= 9 (6 + 51)
= 9 (57)
=513
( 36 - 21 ) : ( 12 - 7 ) = 3
Cari awal :
21 = a + 6b
21 = a + 18
a = 3
Maka, jumlah 18 suku pertama :
S₁₈ = 18 : 2 ( 2 × 3 + ( 18 - 1 ) 3 )
= 9 ( 6 + 17 × 3 )
= 9 × 57
= 513