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Student Support Forum: 'Bug in Mathematica 3 and 4?' topicStudent Support Forum > General > "Bug in Mathematica 3 and 4?"

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Author Comment/Response
Forum Moderator
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09/29/00 7:17pm

>Look at this:
>
>f[k_] := (x /. Solve[6x^2 == k(3x^2 + 3x + 1), x][[2]])
>
>This is just giving f the value of the second solution to the equation.
>
>Now, type
>
>IN: f[k]
>OUT: ''some formatted output for the function f''
>
>Make a new function
>
>g[k_]:= ''copy the output of f[k] here''
>
>Now compare:
>
>IN: f[1] //N
>OUT: 1.26376
>
>IN: g[1] //N
>OUT: -0.263763
>
>See, what is happening is that g is giving the values of the 1st solution to the Solve output, instead of the 2nd as it should be!
>
>I'm not looking for work-arounds for this, as I have already diagnosed the problem and the solution is obvious. I just wanted to point out that mathematica is doing something strange.
>
>Casey Mann
>

========

Not a bug. Look a the results (both parts) of the Solve command for f[k] and f[1] :

In[10]:= f[k_] := (x /. Solve[6x^2 == k(3x^2 + 3x + 1), x])

In[16]:= kfirst=f[k]
Out[16]={(-3*k - Sqrt[3]*Sqrt[8*k - k^2])/(6*(-2 + k)),
(-3*k + Sqrt[3]*Sqrt[8*k - k^2])/(6*(-2 + k))}

In[17]:= kfirst/.k->1
Out[17]= {1/6*(3 + Sqrt[21]), 1/6*(3 - Sqrt[21])}

In[18]:= f[1]
Out[18]= {1/6*(3 - Sqrt[21]), 1/6*(3 + Sqrt[21])}

In your code, Mathematica is delivering the second part in each case.

Tom Zeller
Forum Moderator

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Subject (listing for 'Bug in Mathematica 3 and 4?')
Author Date Posted
Bug in Mathematica 3 and 4? Casey Mann 09/29/00 7:05pm
Re: Bug in Mathematica 3 and 4? Forum Modera... 09/29/00 7:17pm
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