SOLUTION: The sum of 16 terms of an A.P is -504 while the sum of the 9 terms is -126. Find the sum of 30 terms.

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Question 842985: The sum of 16 terms of an A.P is -504 while the sum of the 9 terms is -126.
Find the sum of 30 terms.
Answer by richwmiller(17219) About Me  (Show Source):
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The sum of 16 terms of an A.P is -504 while the sum of the 9 terms is -126.
Find the sum of 30 terms.
S = ½(2a + (n-1)d)n
-504 = ½(2a + 15d)16
-126 = ½(2a + 8d)9
a = 6, d = -5
S = ½(2*6 + -5(29))30
S = -1995 for 30 terms