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# Help? Maths c2 watch

1. maths C2 chapter geometric sequences and series.

Question is:I invest £A in the bank at a rate of interest of 3.5% per annum how long will it be before i double my money
2. (Original post by ballerguids)
maths C2 chapter geometric sequences and series.

Question is:I invest £A in the bank at a rate of interest of 3.5% per annum how long will it be before i double my money
Use the Sum to n formula
You are given that a=A and Sn = 2A and you know r so just need to solve what n is (prob using logs)
3. (Original post by ballerguids)
maths C2 chapter geometric sequences and series.

Question is:I invest £A in the bank at a rate of interest of 3.5% per annum how long will it be before i double my money
Use
Where

and
Then expand, simplify and solve using logs.
4. I have a similar question..

Could someone help me with a Geometric and Sequences question?

2nd term = 16
Sum to infinity = 100

Work out the 2 possible values of the common ration and the corresponding first terms.

Thank you!

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5. (Original post by Annabel_lear)
I have a similar question..

Could someone help me with a Geometric and Sequences question?

2nd term = 16
Sum to infinity = 100

Work out the 2 possible values of the common ration and the corresponding first terms.

Thank you!

Posted from TSR Mobile
A geometric sequence takes the following form

where a is the first term and r is the common ratio.

The sum to infinity is given by

valid for , denoted by , so you can check your answer
6. (Original post by Indeterminate)
A geometric sequence takes the following form

where a is the first term and r is the common ratio.

The sum to infinity is given by

valid for , denoted by , so you can check your answer
I understand that ar=16 and a/1-r = 100
and that you have to solve them by simultaneous equations but I'm not sure how
7. (Original post by Annabel_lear)
I understand that ar=16 and a/1-r = 100
and that you have to solve them by simultaneous equations but I'm not sure how

fill that in and substitute into the other equation
8. (Original post by Indeterminate)

fill that in and substitute into the other equation
I changed ar=16 to a= 16/r
so if you put that into the other equations its
16/r /1-r =100
That's where I get stuck!
9. (Original post by Annabel_lear)
I changed ar=16 to a= 16/r
so if you put that into the other equations its
16/r /1-r =100
That's where I get stuck!

Hint:

10. (Original post by Indeterminate)

Hint:

So does it turn to 16/1-r2 = 100
then
16 = 100 - 100r2 ​?
11. (Original post by Annabel_lear)
So does it turn to 16/1-r2 = 100
then
16 = 100 - 100r2 ​?
Not quite

Notice that you are multiplying by , which is 1, so you're not changing anything.

So

Make sense?
12. (Original post by Annabel_lear)
I understand that ar=16 and a/1-r = 100
and that you have to solve them by simultaneous equations but I'm not sure how
Personally I would start with the 2nd equation which can be rearranged to give a = 100(1-r) and then plug this into the first equation, which gives you:

100r(1-r) = 16

You can then expand this and rearrange to get a quadratic in r.
13. (Original post by Indeterminate)
Not quite

Notice that you are multiplying by , which is 1, so you're not changing anything.

So

Make sense?
Ohhhhh Yupp! Thank you

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