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Asked by  | 9th Mar, 2009, 11:47: AM

Expert Answer:

3 terms in the middle are 10th, 11th and 12th terms. Hence, a+9d +a +10d +a + 11d = 129 ⇒a+10d = 43  ...(1) Last 3 terms are 19th, 20th and 21st terms Hence, a+18d+a+19d+a+20d = 237 ⇒a+19d = 79...(2) Solving (1) and (2) we get a = 3 and d=4 Hence, the AP is 3,7,11,15,19....83  

Aswered by  | 9th Mar, 2009, 02:33: PM

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