Thursday

April 2, 2015

April 2, 2015

Posted by **Sniper** on Sunday, February 27, 2011 at 3:38am.

2) The sum of the first eight terms of a GP is five times the sum of the first four terms. Find the common ratio.

- Geometric -
**agrin04**, Sunday, February 27, 2011 at 4:36am(1) ar^(n-1) + ar^(n-2) + ar^(n-3) = 1024 (a + ar + ar^2)

ar^(n-3) (1 + r + r^2) = 1024a (1 + r + r^2)

r^(n-3) = 1024

It is known that the third term is 5, so: ar^2 = 5

The last term will be:

ar^(n-1) = ar^2. r^(n-3) = 5 x 1024 = ? (You can finish the rest of it, right?)

(2) S8 = 5 S4

a(r^8 - 1)/(r-1) = 5a(r^4 - 1)/(r - 1)

(r^4 - 1)(r^4 + 1) = 5(r^4 - 1)

r^4 + 1 = 5

r^4 = 4

r = ħsqrt(2)

**Answer this Question**

**Related Questions**

Math - 1) If the sum of the first n terms of a series is Sn where Sn=2n²-n, (a)...

math - 1) If the sum of the first n terms of a series is Sn where Sn=2n²-n, (a)...

math - in an arithmetic sequence the common difference is equal to 2.the first ...

arithmetic - 1. The first and last term of an A.P are, a and l respectively, ...

arithmetic - 1. The first and last term of an A.P are, a and l respectively, ...

Maths - 1..The first 2 terms of a geometric progression are the same as the ...

math - in a geometric sequence the first term is 2. The 3rd term is the sum of ...

arithmetic - An arithmetic n geometric sequence have the same first term 2.with ...

Math:A.P. - The 1st,3rd and 9th terms of an A.P are the three terms of a G.P.if ...

Math *URGENT - Please give the answers and solutions for each. 1.If the second ...