Jawabanmu

2014-04-20T00:17:25+07:00
U5 = a + (4)b = 36 u12 = a + (11)b =15 => -7b = 21 => b = 21/7 b = 3 .u5 = a +(4)3 =36 u5 = a + 12 = 36 a = 36/12 = 3 => u23 = 3 + 22.3 = 3 + 66 =69
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2014-04-20T00:19:55+07:00
Pertama, cari bnya dulu, caranya:
b = U12-U5 ÷ 12-5
=15-36 ÷7
= -21 ÷ 7
= -3
Terus, dari suku ke 12 dicari a nya:
Un = a + (n-1)b
U12 = a + (12-1) x (-3)
15 = a + 11 x (- 3)
15 = a + (-33)
15+33 = a
48 = a

Jadi, suku ke-23nya:
U23 = a + (23-1)b
= 48 + 22 x (-3)
= 48 + (-66)
= -18