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Hello, I think the answer for

http://www.mathopolis.com/questions/q.php?id=589

Have a typo.

Q : 3 years ago you had $120 in the bank. You now have $192.

If the amount of money at the end of each year increases exponentially, then how much will you have in the bank after another 7 years?

(Assume that you do not deposit or withdraw any money)

A : Start with the formula y(t) = aekt

We know y(0) = 120

⇒ 120 = ae0

⇒ a = 120

We also know y(3) = 192

⇒ 192 = 120e3k

⇒ e3k = 192 ÷ 120 = 1.6

⇒ 3k = ln(1.6) = 0.470...

⇒ k = 0.470... ÷ 3 = 0.156668...

Now we know a and k, we can write the growth function y(t) = aekt explicitly as

y(t) = 120e0.156668t

Now, we can find how much money there will be in the bank after a further 6****(isn't this 7) years by replacing t with 10:

y(10) = 120e0.156668 × 10

= 120e1.56668

= 120 × 4.79071658...

= 574.89

You will have $574.89 in the bank after a further 7 years

Sorry if I was wrong, please explain.

It's not how good you are. It's how good you want to be - Paul Arden

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 84,195

Hi;

My #589 is about rabbits.

**In mathematics, you don't understand things. You just get used to them.I have the result, but I do not yet know how to get it.All physicists, and a good many quite respectable mathematicians are contemptuous about proof.**

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**ShivamS****Member**- Registered: 2011-02-07
- Posts: 3,514

Chooipian, your answer is right.

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 84,195

Hi chooipian;

Please learn to use latex.

http://www.codecogs.com/latex/eqneditor.php

And find the correct link to the question that I may edit it.

**In mathematics, you don't understand things. You just get used to them.I have the result, but I do not yet know how to get it.All physicists, and a good many quite respectable mathematicians are contemptuous about proof.**

**Online**

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**bob bundy****Moderator**- Registered: 2010-06-20
- Posts: 6,294

It is the second question not the first.

http://www.mathopolis.com/questions/q.php?id=589&site=1&ref=/algebra/exponential-growth.html&qs=589_1238_1239_2304_2305_8089_8090_8091_8092

edit: If you put the second page link in, you get the first question. Does that always happen or is there a bug?

and chooipian's answer is correct. I chose answer C and was told I'm correct with that answer.

Bob

*Last edited by bob bundy (2014-05-29 01:37:42)*

You cannot teach a man anything; you can only help him find it within himself..........Galileo Galilei

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 84,195

Take what codecogs spits out and put it between

`[math]Put math here.[/math]`

Hi Bob;

If you put the second page link in, you get the first question. Does that always happen or is there a bug?

Looks like a bug.

**In mathematics, you don't understand things. You just get used to them.I have the result, but I do not yet know how to get it.All physicists, and a good many quite respectable mathematicians are contemptuous about proof.**

**Online**

**bob bundy****Moderator**- Registered: 2010-06-20
- Posts: 6,294

hmmm. id = 1238 leads to question 2 but my screen has question 2 and id = 589.

I've not played with these questions so I've not tested this before.

maybe chooipian got directed to the wrong answer link.

Bob

*Last edited by bob bundy (2014-05-29 01:42:46)*

You cannot teach a man anything; you can only help him find it within himself..........Galileo Galilei

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 84,195

I am getting C, so what is the problem?

I have the result, but I do not yet know how to get it.

All physicists, and a good many quite respectable mathematicians are contemptuous about proof.

**Online**

Now, we can find how much money there will be in the bank after a further 6 years by replacing it with 10:

It should be 7 not 6.

*Last edited by chooipian (2014-05-29 02:17:03)*

It's not how good you are. It's how good you want to be - Paul Arden

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 84,195

I got it! Thanks for spotting it.

I have the result, but I do not yet know how to get it.

All physicists, and a good many quite respectable mathematicians are contemptuous about proof.

**Online**