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    Question

    In an arithmetic progression, the 4th term is 23 and the

    9th term is 48. What is the sum of the first 20 terms?
    A 1160 Correct Answer Incorrect Answer
    B 1025 Correct Answer Incorrect Answer
    C 1110 Correct Answer Incorrect Answer
    D 1250 Correct Answer Incorrect Answer

    Solution

    ATQ, Let first term = a, common difference = d. 4th term: a + 3d = 23 9th term: a + 8d = 48 Subtract: (a + 8d) тИТ (a + 3d) = 48 тИТ 23 5d = 25 тЗТ d = 5. Then a = 23 тИТ 3d = 23 тИТ 15 = 8. Sum of first 20 terms: SтВЩ = n/2 [2a + (n тИТ 1)d] SтВВтВА = 20/2 [2┬╖8 + 19┬╖5] = 10 [16 + 95] = 10 ├Ч 111 = 1110.

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