Re: plot command

*To*: mathgroup at smc.vnet.net*Subject*: [mg58151] Re: plot command*From*: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>*Date*: Mon, 20 Jun 2005 05:21:25 -0400 (EDT)*Organization*: The Open University, Milton Keynes, England*References*: <d90spv$8va$1@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

Bosch, Darrell wrote: > Dear Group: I am trying to solve an equation, take the derivative and > plot the derivative as shown below. I get a message below the Plot > request stating > > "Syntax::sntxi: "Incomplete expression; more input is needed." Any help > would be greatly appreciated. Darrell Bosch > > > > > > > > sol = Log[v]=A9100*Log(s) > > sol2 = Solve[sol,v] > > sol3 = D[sol2,s] > > Plot[Evaluate[sol3],{s,0,100}] > > > > Hi Darrell, Using correct syntax would help greatly! I have not gotten the error message you mentioned in your post but definitely many others. Starting from the first line, we get In[1]:= sol = Log[v] = A9100 Log(s) Set::write : Tag Log in Log[v] is Protected. Out[1]= A9100*Log*s Here you are using the symbol "=" (simple equal), which is equivalent to the command *Set*, which tries to assign a new value/definition to the built-in command *Log*. So, either you *Unprotect* the *Log* function (very bad idea in this case :-) or most likely you use the *Equal* (double =, that is "==") to check for equality between LHS and RHS. Of course you still use the simple "=" to assign the equation to the symbol "sol". Moreover, as you can see above, Log(s) is translated as Log*s, that is Log TIMES s, surely not what you wanted. Here too, using the correct syntax would help. What you want is Log[s] (square brackets). You may have a look at http://documents.wolfram.com/mathematica/GettingStarted/StartingOut/YourFirstMathematicaCalculations.html Therefore, what is the correct syntax for the first line (assuming that "A9100" is the symbol A9100 and not a misspelling for A*9100 or whatever else)? In[2]:= sol = Log[v] == A9100*Log[s] Out[2]= Logs[v] == A9100*Log[s] Now the second line gives In[3]:= sol2 = Solve[sol, v] Out[3]= {{v -> s^A9100}} a replacement rule rather than the error message "Solve::eqf : A9100\Log\s is not a well-formed equation." Now, if you derive directly this replacement rule, you get In[4]:= sol3 = D[sol2, s] Out[4]= {{0 -> A9100*s^(-1 + A9100)}} which is not what you want: LHS and RHS are both derived and since "v" does not depend explicitly on "s" its derivative is zero. One of the possible solutions among many variations is In[6]:= sol3 = D[v /. sol2[[1]], s] Out[6]= A9100*s^(-1 + A9100) Finally, the last command yields an error message In[7]:= Plot[Evaluate[sol3], {s, 0, 100}] \!\(Plot::"plnr" : "\!\(A9100 \\\\ s\^\(\(-1\) + A9100\)\) is not a machine-size real number at s = \ 4.166666666666667`*^-6."\) telling us that A9100 is not a number, which is true. We can circumvent this "problem" by assigning a value to A9100 before plotting the function: In[7]:= Plot[Evaluate[sol3 /. A9100 -> 1], {s, 0, 100}]; And the magic operates: we get a superb graph (of a straight line in this case :-) Best regards, /J.M.