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An arithmetic progression has 11 as its first term and fourth term is 32.find the sum of the first nine terms.

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3 differences between 1st and 4th terms ... d = (32 - 11) / 3 = 7

9th term = 1st term + 8 d = 67

sum of 1st 9 terms ... (11 + 67) * (9/2)

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