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If the 1st term of a series is 7 and 13th term is 35. Find the sum of 13 terms of the sequence. |
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Answer» Given a = 7, a13 = 35 We find to find d We know that an = a + (n - 1)d Putting a = 7, n = 13 and an = 35 35 = 7 + (13 - 1) x d 35 = 7 + 12d 35 - 7 = 12d 28 = 12 d \(\frac{28}{12}=d\) \(\frac73=d\) d = \(\frac73\) Now we need to find S13 We can use formula Sn = \(\frac n2(a+l)\) Putting n = 13, a = 7, a13 = 35 = \(\frac{13}2\)(7 + 35) = \(\frac{13}2\) x 42 = 13 x 21 = 273 |
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