Homework Help: Math

Posted by Anonymous on Thursday, October 27, 2011 at 10:14am.

The sum of 16terms oe an A.P is -504,while the sum of its 9terms is -126.Find the sum of its 30terms

Math - Reiny, Thursday, October 27, 2011 at 10:32am
sum of 16 terms is -504 ---> 8(2a + 15d) = -504
2a + 15d = -63

sum of 9 terms is -126 --> (9/2)(2a + 8d) = -126
2a + 8d = -28

subtract:
7d = -35
d=-5
in 2a+8d=-28
2a - 40 = -28
2a = 12
a=6

sum(30) = 15(12 + 29(-5)) = -1995
Math - Steve, Thursday, October 27, 2011 at 10:37am
S₁₆ = 16/2 * (2a+15d) = -504
S₉ = 9/2 * (2a + 8d) = -126

16a + 120d = -504
9a + 36d = -126

Multiply #1 by 9 and #2 by 16, then subtract:

144a + 1080d = -4536
144a + 576d = -2016

504d = -2520
d = -5
a = 6

S₃₀ = 15(2a+29d) = 15(12 - 145) = -1995
Math - Obagu, Sunday, October 7, 2012 at 7:43pm
-1995

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