Tentukan U4 dari deret aritmatika yang rumus jumlah n suku pertamanya adalah Sn = 2n²-2

Sn = 2n² - 2
S1 = 2(1)² - 2 = 2 - 2 = 0
S2 = 2(2)² - 2 = 2(4) - 2 = 8 - 2 = 6

S1 = a = 0
S2 = a + b = 6
0 + b = 6
b = 6

U4 = a + 3b
= 0 + 3(6)
= 0 + 18
= 18

sn = n/2 (a + un)
a = s1
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s1 = 2(1²) - 2
s1 = 0

s4 = 4/2 (0 + u4)
2(4²) - 2 = 2u4
30 = 2u4
u4 = 30/2
u4 = 15