# Several errors from copied NDSolve code

**URL:** <https://community.wolfram.com/t/several-errors-from-copied-ndsolve-code/23307>\
**Category:** Questions\
**Tags:** calculus, equation-solving, graphics-and-visualization, numerical-computation\
**Created:** [December 25, 2023, 1:20pm UTC](https://community.wolfram.com/t/several-errors-from-copied-ndsolve-code/23307 "2023-12-25T13:20:17Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![mohsin](https://avatars.discourse-cdn.com/v4/letter/m/41988e/32.png) [@mohsin](https://community.wolfram.com/u/mohsin)\
**Post date:** [December 25, 2023, 1:20pm UTC](https://community.wolfram.com/t/several-errors-from-copied-ndsolve-code/23307/1 "2023-12-25T13:20:17Z")

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Can anyone help me with this code, I copied it from some textbook but it is not working the way it

[Open in Wolfram Cloud ↗](https://www.wolframcloud.com/obj/8e3f8759-0ec5-42b7-aa89-69d13251dc1c)

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**Author:** ![dontdont9](https://avatars.discourse-cdn.com/v4/letter/d/54ee81/32.png) [@dontdont9](https://community.wolfram.com/u/dontdont9)\
**Post date:** [December 25, 2023, 1:56pm UTC](https://community.wolfram.com/t/several-errors-from-copied-ndsolve-code/23307/2 "2023-12-25T13:56:46Z")

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Try the following changes

```
k1f=0.5;k1r=0.2;k2f=0.5;k2r=(k1r/k1f)*k2f;k3f=1.5;k3r=10^-3;tmax=1;
S=1.25;T=S;P=0.5;
R1f:=k1f*S*T;R1r:=k1r*XL[t];
R2f:=k2f*S*T*XL[t];R2r:=k2r*(XL[t])^2;
R3f:=k1f*S*T;R3r:=k1r*XD[t];
R4f:=k2f*S*T*XD[t];R4r:=k2r*(XD[t])^2;
R5f:=k3f*XL[t]*XD[t];R5r:=k3r*P;
Soln1=NDSolve[{XL'[t]\[Equal]R1f-R1r+R2f-R2r-R5f+R5r,XD'[t]\[Equal]R3f-R3r+R4f-R4r-R5f+R5r,XL[0]\[Equal]0.2502,XD[0]\[Equal]0.2500},{XL[t],XD[t]},{t,0,2000},MaxSteps->10000];
Plot[Evaluate[{XL[t],XD[t]}/.Soln1],{t,0,tmax},AxesLabel->{"time","[XL]&[XD]"},AxesStyle->Directive[Black,14],PlotStyle->{{Black,Thick},{Gray,Thick}},PlotRange->All]

```

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<div class="post-metadata">

**Author:** ![mohsin](https://avatars.discourse-cdn.com/v4/letter/m/41988e/32.png) [@mohsin](https://community.wolfram.com/u/mohsin)\
**Post date:** [December 26, 2023, 11:33am UTC](https://community.wolfram.com/t/several-errors-from-copied-ndsolve-code/23307/3 "2023-12-26T11:33:29Z")

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Dear Bill Nelson,  
Thank you so much! it really works! Can you suggest how i can put animate command in this code so that my output is an animation with respect to variable t in the code

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<div class="post-metadata">

**Author:** ![dontdont9](https://avatars.discourse-cdn.com/v4/letter/d/54ee81/32.png) [@dontdont9](https://community.wolfram.com/u/dontdont9)\
**Post date:** [December 26, 2023, 4:04pm UTC](https://community.wolfram.com/t/several-errors-from-copied-ndsolve-code/23307/4 "2023-12-26T16:04:38Z")

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Try

```
k1f=0.5;k1r=0.2;k2f=0.5;k2r=(k1r/k1f)*k2f;k3f=1.5;k3r=10^-3;
S=1.25;T=S;P=0.5;
R1f:=k1f*S*T;R1r:=k1r*XL[t];
R2f:=k2f*S*T*XL[t];R2r:=k2r*(XL[t])^2;
R3f:=k1f*S*T;R3r:=k1r*XD[t];
R4f:=k2f*S*T*XD[t];R4r:=k2r*(XD[t])^2;
R5f:=k3f*XL[t]*XD[t];R5r:=k3r*P;
Animate[
Soln1=NDSolve[{XL'[t]\[Equal]R1f-R1r+R2f-R2r-R5f+R5r,XD'[t]\[Equal]R3f-R3r+R4f-R4r-R5f+R5r,XL[0]\[Equal]0.2502,XD[0]\[Equal]0.2500},{XL[t],XD[t]},{t,0,2000},MaxSteps->10000];
Plot[Evaluate[{XL[t],XD[t]}/.Soln1],{t,0,tmax},AxesLabel->{"time","[XL]&[XD]"},AxesStyle->Directive[Black,14],PlotStyle->{{Black,Thick},{Gray,Thick}},PlotRange->{{0,.9},{.25,.75}}],
{tmax,0,1}]

```

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<div class="post-metadata">

**Author:** ![mohsin](https://avatars.discourse-cdn.com/v4/letter/m/41988e/32.png) [@mohsin](https://community.wolfram.com/u/mohsin)\
**Post date:** [December 26, 2023, 4:10pm UTC](https://community.wolfram.com/t/several-errors-from-copied-ndsolve-code/23307/5 "2023-12-26T16:10:44Z")

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Dear Bill Nelson,  
Thank you so much. I never had imagined Mathematica can do this, hope my students will enjoy this! Thank you once again