# algorithms

=?Utf-8?B?TGVtYml0?=
Guest
Posts: n/a

 1st Feb 2005
I need help!!!
What should i do if i want to get out r from this function where i already
have a and b.

(1-(1/(1+r))^b)/r=a is there any logarithm and if there is in what library
do i find it

Fredrik Wahlgren
Guest
Posts: n/a

 1st Feb 2005

"Lembit" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> I need help!!!
> What should i do if i want to get out r from this function where i already
> have a and b.
>
> (1-(1/(1+r))^b)/r=a is there any logarithm and if there is in what library
> do i find it

I think this is correct. Check it.

(1-(1/(1+r))^b)/r=a
(1-(1/(1+r))^b)=a*r
1-(1/(1+r)) = (a*r)^(1/b)
-1/(1+r) = (a*r)^(1/b)-1
1/(1+r) = -((a*r)^(1/b)-1)
1+r = 1/(-((a*r)^(1/b)-1))
r = 1/(-((a*r)^(1/b)-1))-1

/Fredrik

Fredrik Wahlgren
Guest
Posts: n/a

 1st Feb 2005

"Fredrik Wahlgren" <(E-Mail Removed)> wrote in message
news:%(E-Mail Removed)...
>
> "Lembit" <(E-Mail Removed)> wrote in message
> news:(E-Mail Removed)...
> > I need help!!!
> > What should i do if i want to get out r from this function where i

> > have a and b.
> >
> > (1-(1/(1+r))^b)/r=a is there any logarithm and if there is in what

library
> > do i find it

>

Carl Daniel [VC++ MVP]
Guest
Posts: n/a

 2nd Feb 2005
Lembit wrote:
> I need help!!!
> What should i do if i want to get out r from this function where i
> already have a and b.
>
> (1-(1/(1+r))^b)/r=a is there any logarithm and if there is in what
> library do i find it

I'm too lazy to attempt the algebra at the moment, but looking at it, I'd
say no, there is no formula for that value - you'll have to use numerical
methods to search for a suitable r for a given a and b. For example, you
might use "Newton's Method" to solve this. You'll likely have to look it up
in a numerical methods book or a calculus book.

-cd

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