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Un = a + (n-1)b
maka
U5 adalah
a + 4b = 11
5 + 4b = 11
4b = 11 - 5
b = 6/4
= 1,5
Sn = n/2 (2a + (n-1)b)
S20 = 20/2 (2x5 + 19x1,5)
= 10 (10+38,5)
= 10 x 48,5
= 485
Jadi jumlah suku 20 pertamannya yaitu 485