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Student Support Forum: 'Minimum of a two-argument function' topicStudent Support Forum > General > "Minimum of a two-argument function"

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Author Comment/Response
Bill Simpson
09/26/12 1:59pm

Use Reduce to solve for R

In[1]:= Reduce[700000*2*R/(Pi*(R^4-r^4))==318&&R>r>0,R]

Out[1]= r > 0 && R == Root[-159*Pi*r^4 - 700000*#1 + 159*Pi*#1^4 & , 2]

Scrape that Root object and paste into ToRadicals

In[2]:= x = ToRadicals[Root[-159*Pi*r^4 - 700000 #1 + 159*Pi*#1^4 &, 2]];

Try to minimize

In[3]:= NMinimize[{x,r>0},r]

From In[3]:= NMinimize::nrnum: The function value -5.59526+9.69128I is not a real number at {r} = {0.0637073}

and your function is complex in some places.

Table[r = RandomReal[{0, 1000}]; {r, x}, {100}]

seems to show me that it is complex lots of places and perhaps almost everywhere.

Is there some mistake in something I've done or can you perhaps revise your question so that a domain where it is real can be isolated?

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Subject (listing for 'Minimum of a two-argument function')
Author Date Posted
Minimum of a two-argument function Rifi 09/26/12 09:58am
Re: Minimum of a two-argument function Bill Simpson 09/26/12 1:59pm
Re: Re: Minimum of a two-argument function Rifi 09/27/12 07:27am
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