Re: Using NIntegrate[] within FindRoot[] ??

• To: mathgroup at smc.vnet.net
• Subject: [mg2328] Re: Using NIntegrate[] within FindRoot[] ??
• From: wagner at goober.cs.colorado.edu (Dave Wagner)
• Date: Tue, 24 Oct 1995 02:15:11 -0400
• Organization: University of Colorado, Boulder

```In article <DGvx8w.B2J at wri.com>,
Axel Kowald <a_kowald at chemie.fu-berlin.de> wrote:
>
>Hello everybody
>
>
>I like to use NIntegrate with FindRoot as in the following trivial example:
>
>            FindRoot[NIntegrate[r x,{x,0,3}] == 9, {r,1}]
>
>That means I want to find the value of "r" for which the integral is equal to 9.
>However, I get the error message:
>
>                Integrand 1.5 r is not numerical ...
>
>What am I doing wrong and how am I doing it right ???

You are using FindRoot on an expression whose symbolic derivative with
respect to r cannot be computed.  To get around this problem, give
FindRoot two initial points, rather than one, so that it uses the
secant method of root-finding.

In[6]:=
FindRoot[NIntegrate[r x,{x,0,3}] == 9, {r,1,1.1}]
Out[6]=
{r -> 2.}

Dave Wagner
Principia Consulting
(303) 786-8371
dbwagner at princon.com
http://www.princon.com/princon

```

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