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Verified answer
Jawabderet
u6 = 15
u13 = 43
u13 - u6 = 43 - 15
(13-6)b = 28
7b = 28
b = 4
u6 = 15
a + 5b = 15
a + 20 = 15
a = - 5
Sn = n/2 ( 2a + (n-1)b))
s20 = 20/2 ( 2(-5) + 19(4)
s20 =10 ( -10 + 76)
s20 = 660