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U1. = U7- B x 6
= 15 - 12 = 3 , U21 = U1 + B x 20 , = 3 + 40 = 43
u7 = a + (n-1) b = 15 | un = a + (7-1) b = 15
a + 2b = 7
a + 6b = 15
-------------- - (dikurang)
-4b = -8
b = -8/-4 = 2
b = 2
nyari a =
a + 2b = 7
a + 2.2 = 7
a + 4 = 7
a = 7-4
a = 3
nyari Suku 21 =
sn = n/2 (2a + (n-1) b )
s21 = 21/2 (2.3 + (21-1) 2 )
s21 = 21/2 (6 + 40)
s21 = 21/2 (46)
s21 = 483