Sunday

April 20, 2014

April 20, 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.

**Related Questions**

math - 1) If the sum of the first n terms of a series is Sn where Sn=2nē-n, (a)...

Math - The sum of the first n terms of an arithmetic series is given by: Sn=5/2n...

pre calc - If Sn represents the sum of the squares of the first n natural ...

math 12 - sequences and series - Given the expression Sn = n^2 + 2n for the sume...

C++ - Write a MAIN function and a FUNCTION to compute the sum of the below ...

math - thinking/inquiry/problem solving: a formula for the sum of the first n ...

Pre Calc - Write as a series and express as a rational number: 1. 5.36363636...

C++ - Write a MAIN function and a FUNCTION to compute the sum of the below ...

C++ - Write a MAIN function and a FUNCTION to compute the sum of the below ...

C++ - you are right the question is exactly like this... can you help me for ...