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Verified answer
Sn = n^2 + nU4 = S4 - S(4-1)
U4 = S4 - S3
U4 = (4^2 + 4) - (3^2 + 3)
U4 = (16 + 4) - (9 + 3)
U4 = 20 - 12
U4 = 8