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{SECT 0 {EXCHG {PARA 0 "" 0 "" {TEXT -1 80 "The solution of the equati
on is a fundamental solution of the diffusion equation" }}{PARA 0 "> \+
" 0 "" {MPLTEXT 1 0 58 "Phi:=(t,x)->(1/(sqrt(4*Pi*0.04*t)))*exp(-x^2/(
4*0.04*t)); " }}}{EXCHG {PARA 0 "" 0 "" {TEXT -1 56 "We try to find at
 what time, the concentration at x=200 " }}{PARA 0 "> " 0 "" {MPLTEXT 
1 0 22 "derivative:=D[1](Phi);" }}}{EXCHG {PARA 0 "> " 0 "" {MPLTEXT 
1 0 41 "derivative(t,200); plot(%, t=0..1000000);" }}}{EXCHG {PARA 0 "
> " 0 "" {MPLTEXT 1 0 30 "fsolve(derivative(t,200)=0,t);" }}}{EXCHG 
{PARA 0 "> " 0 "" {MPLTEXT 1 0 21 "Phi(%,200); evalf(%);" }}}{EXCHG 
{PARA 0 "" 0 "" {TEXT -1 63 "Now we try to find when the concentration
 will be 0.00054 again" }}{PARA 0 "> " 0 "" {MPLTEXT 1 0 41 "con:=Phi(
t,200); plot(con,t=0..10000000);" }}}{EXCHG {PARA 0 "> " 0 "" 
{MPLTEXT 1 0 48 "fsolve(con=0.00054,t, 150000..10000000); %/3600;" }}}
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