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# C2 geometric series question watch

1. The first three terms of a geometric series are 4p, (3p + 15) and (5p + 20) respectively,where p is a positive constant.

Show that 11p2 – 10p – 225 = 0

Eh?
2. (Original post by jessyjellytot14)
The first three terms of a geometric series are 4p, (3p + 15) and (5p + 20) respectively,where p is a positive constant.

Show that 11p2 – 10p – 225 = 0

Eh?
Well, what does your gut instinct tell you to do?

Hint:
Spoiler:
Show
it has something to do with 'r', the common ratio.
3. for any GP there is a way of finding the common ratio r using consecutive terms...
4. (Original post by jessyjellytot14)
The first three terms of a geometric series are 4p, (3p + 15) and (5p + 20) respectively,where p is a positive constant.

Show that 11p2 – 10p – 225 = 0

Eh?
What do you know about consecutive terms in a geometric series? They have a common ratio.

The ratio of the second term to the first is the same as the ratio of the third term to the second, etc...

So . Here we have: , now multiply out and simplify into the given quadratic.
5. (Original post by SeanFM)
Well, what does your gut instinct tell you to do?

Hint:
Spoiler:
Show
it has something to do with 'r', the common ratio.
I'll try doing ar/a = ar2/ar
6. (Original post by jessyjellytot14)
I'll try doing ar/a = ar2/ar
Update: It worked
7. (Original post by jessyjellytot14)
Update: It worked
Well done!

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