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U3 = a + 2b
a+ 2b = 18
U7 = a + 6b
a + 6b = 38
pakai cara eliminasi
a + 6b = 38
a + 2b = 18
eliminasikan a
4b = 20
b = 5
cari a
a + 2b = 18
a + 2.5 = 18
a = 18 - 10
a = 8
Sn = n/2 (2a + (n-1)b)
= 24/2 (2.8 + 23.5)
= 12 (16 + 115)
= 12 x 131
= 1572
jawaban: c
u7=38
u3=a+2b⇒18=a+2b
u7=a+6b⇒38=a+6b _
-20=-4b
b=5
u3=a+2b
18=a+2(5)
18=a+10
a=8
u1=8
u24=8+23(5)
=8+115
=123
u1+u24=8+123
= 131
131x12=1572