The sum of 16 terms of an AP is -504
gives us: 8(2a + 15d) = -504
2a + 15d = - 63
the sum of its 9 terms is -126
gives us: (9/2)(2a + 8d) = -126
2a + 8d = - 28
subtract them,
7d = - 35
d = - 5 , then a = 6
sum(30) = 15(-10 + 29(6)) = ....
The sum of 16 terms of an AP is -504, while the sum of its 9 terms is -126. Find the sum of its 30 terms
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How did you get 6