Thursday

August 21, 2014

August 21, 2014

Posted by **Anonymous** on Thursday, April 17, 2008 at 2:32pm.

(a)prove that the series is an AP, stating the first term and the common difference;

(b)find the sum of the terms from the 3rd to the 12th inclusive

2) In an AP the 6th term is half the 4th term and the 3rd term is 15.

(a)Find the first term and the common difference;

(b)How many terms are needed to give a sum that is less than 65?

Please help

- Math -
**Reiny**, Thursday, April 17, 2008 at 3:01pmIsn't t(n) = S(n) - S(n-1)

(e.g. the sixth term is sum of 6 terms minus the sum of 5 terms)

so t(n) = 2n^2-n - (2(n-1)^2 - (n-1))

= 4n - 3

so from that you can now find any of the terms

notice t(1) = a = 1

d = 4

2) "the 6th term is half the 4th term"

----> a+5d = 1/2(a+3d)

"the 3rd term is 15" -----> a+2d = 15

solve these two equations.

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