# New Spec A Level Sequences

Watch
Announcements

Page 1 of 1

Go to first unread

Skip to page:

The first three terms of a geometric sequence are:

k+2, 4k, 2k^2, k>0

Find the value of k.

I got k=10 after setting k+2+4k equal to the Sn equation? I worked out r as k/2.

Is this right? Thank you!

k+2, 4k, 2k^2, k>0

Find the value of k.

I got k=10 after setting k+2+4k equal to the Sn equation? I worked out r as k/2.

Is this right? Thank you!

0

reply

Report

#2

(Original post by

The first three terms of a geometric sequence are:

k+2, 4k, 2k^2, k>0

Find the value of k.

I got k=10 after setting k+2+4k equal to the Sn equation? I worked out r as k/2.

Is this right? Thank you!

**IzzyLJB**)The first three terms of a geometric sequence are:

k+2, 4k, 2k^2, k>0

Find the value of k.

I got k=10 after setting k+2+4k equal to the Sn equation? I worked out r as k/2.

Is this right? Thank you!

Meaning you did it wrong.

Why not just make the ratios between consecutive terms the same since that must be true on a geo sequence??

I.e.

0

reply

Report

#3

Sorry, just found this thread and I’m having problems with the same question. I’d be fine if I could find the common ratio. How can I find the common ratio for that question?

0

reply

Report

#4

(Original post by

Sorry, just found this thread and I’m having problems with the same question. I’d be fine if I could find the common ratio. How can I find the common ratio for that question?

**Ncodling2017**)Sorry, just found this thread and I’m having problems with the same question. I’d be fine if I could find the common ratio. How can I find the common ratio for that question?

0

reply

Report

#5

**Ncodling2017**)

Sorry, just found this thread and I’m having problems with the same question. I’d be fine if I could find the common ratio. How can I find the common ratio for that question?

(Original post by

I think RDKGames gives you the ratio for the first and second two terms and makes them equal (which it must be for a geometric progression). Simply solve that equation k and check it works.

**mqb2766**)I think RDKGames gives you the ratio for the first and second two terms and makes them equal (which it must be for a geometric progression). Simply solve that equation k and check it works.

**IzzyLJB**)

The first three terms of a geometric sequence are:

k+2, 4k, 2k^2, k>0

Find the value of k.

I got k=10 after setting k+2+4k equal to the Sn equation? I worked out r as k/2.

Is this right? Thank you!

*a*(k+2 in this question)

Second term is

*ar*(4k in this question)

Third term is

*ar^2*(2k^2)

Therefore, using basic power laws, and cancelling stuff

ar/a = ar^2 / ar (cancels to r = r)

so

4k / (k+2) = 2k^2 / 4k

Then you simply solve the equation you have formed to find k.

This is very similar to Question 9 on the 2017 C2 paper (old spec) for Edexcel. Have a go at that. I sat that paper.

2

reply

X

Page 1 of 1

Go to first unread

Skip to page:

### Quick Reply

Back

to top

to top