Differential equations interest probelm

atrus_ovis
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Homework Statement


Two same aged people P1,P2 create bank accounts at ages 25,35 respectively, and add 2000 euros annualy , for 10,30 years respectively. No initial capital in the accounts.

1.For interest 8% , what's the balance of each acount at age of 65 of P1,P2?
2.What should be the interest for the above balances to be equal

The Attempt at a Solution


I guess continuous interest is assumed.

The problem here is the constant 2000/year.
The interest r affects the balance in the form of
S_r(t) = S_0e^{rt} , while the constant deposits S_{c0}(t) = S_{c0} + 2000t

edit : To the result of the bank's interest over a year, 2000 is added and the total is considered the balance for the application of interest at the next year.
Does it hold to say:
\frac{dS}{dt} = (2000+S)r \iff S(t) = S_0 e^{rt} + 2000rt ?

Do i have to combine them somehow / write the relation as a DE and find the solution?
hints?
 
Last edited:
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I think i got it:
<br /> S(t) = e^{rt}+2000rt+2000t<br />
 
There are two things I don't understand about this problem. First, when finding the nth root of a number, there should in theory be n solutions. However, the formula produces n+1 roots. Here is how. The first root is simply ##\left(r\right)^{\left(\frac{1}{n}\right)}##. Then you multiply this first root by n additional expressions given by the formula, as you go through k=0,1,...n-1. So you end up with n+1 roots, which cannot be correct. Let me illustrate what I mean. For this...

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