No 4 gimna cara nya, tolong bantu, besok dikumpulkan

Sn = 1/3 (n + 3)^2
U5 = S5 - S4
= 1/3 (5 + 3)^2 - 1/3 (4 + 3)^2
= 1/3 (8)^2 - 1/3 (7)^2
= 64/3 - 49/3
= 15/3
= 5
U2 = S2 - S1
= 1/3 (2 + 3)^2 - 1/3 (1 + 3)^2
= 1/3 (5)^2 - 1/3 (4)^2
= 25/3 - 16/3
= 9/3
= 3
U5 + U2 = 5 + 3 = 8