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6 = 3a + 3b
2 = a+b
a = 2-b
u10+u11+u12 = a +9b + a+10b +a +11b
33 = 3a +30b
11 =a +10b
11 = 2 - b +10b
9 = 9b
b = 1
a= 2 -b=2-1 = 1
suku pertama = 1
jumlah 12 suku = 12/2 (2x1 + (12-1)1)
= 6 (2 + 11)
= 6 x 13
= 78