Here, a = 7, a13 =35, find d and S13. We know that, an = a + (n - 1)d a13 = 7 + (13 - 1)d 35 = 7 + 12d 35 - 7 = 12d 12d = 28
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65.
In an AP Given a12 = 37, d = 3, find ‘a’ and S12.
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66.Given a3 = 75, S10 = 120, find ‘d’ and a10.
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67.
In an AP Given d = 5, 59 = 75, find ‘a’ and ‘a’
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68.Find the sum of the series 5 + 7 + 9+ 10 + 13 + 13 + 17 + 16 +.........to 40 terms.
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69.Find four numbers in A .P. whose sum is 20 and the sum of whose squares is 180.
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70.Prove that no matter what the real numbers ‘a’ and ‘b’ are the sequence with nth term a + nb is always an A .P. what is the common difference ? What is the sum of the first 20 terms.