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a = U4 - 3b = 15 - 3(3) = 15 - 9 =6
S20 = n/2 (2a + (n-1)b) = 20/2 ( 2(6) +(20-1)3) = 10 (12 +57) = 10 (69) = 690
nb = tanda / berarti per atau bagi :D