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U4 + U16 = a + 3b + a + 15b = 2a + 18b = 28 ...(2)
eliminasi (1) & (2), didapat:
a=U1= 31/8
b = 9/8
a. U15 = a + 14b = 9/8 + 14.9/8 = 157/8 = 19 5/8
b. Sn = n/2 (U1 + U15)
S15 = 15/2 (31/8 + 157/8) = 7,5(188,8) = 176,25 = 176 1/4