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The sum of 18 terms of an A.P is 504 while the sum of its 9 terms is -126.Find the sum of its 30 term

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I will assume you know the formulas for the sum of n terms of an AP
Sum(18) = 9(2a + 17d) = 504
2a + 17d = 56

Sum(9) = (9/2)(2a + 8d) = -126
2a + 8d = -28

subtract them: 9d =84
d = 28/3
sub into 2a+8d=-28
2a + 224/3 = -28
a = -154/3

sum(30) = 15(2a + 29d) = .....
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