Bantu jawabb dongg Deret aritmatika u3=18, u17=26 a. Tentukan a dan b b. Un c. Jumlah keduabelas suku pertama

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U3 = a + 2b = 18
U17 = a + 16b = 26

a + 16b = 26
a + 2b = 18
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14b = 8
b = 8/14
b = 4/7

a + 2b = 18
a + 2(4/7) = 18
a + 8/7 = 18
a = 18 - 8/7
a = 126/7 - 8/7
a = 118/7

Un = a + bn - b
Un = 118/7 + 4/7n - 4/7
Un = 114/7 + 4/7n

Sn = ½ n ( a + Un )
S12 = ½ (12) ( 118/7 + 114/7 + 4/7×12)
S12 = (6) ( 118/7 + 114/7 + 48/7)
S12 = 6 × 280/7
S12 = 6 × 40
S12 = 240

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