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کد:
u:=sum((c[n]*cos(lambda[n]*t)+d[n]*sin(lambda[n]*t))*sin(n*Pi*x/L),n=1..40);
   (1.117372821 + 1.220847109 sinx cosx) sin(x)

      + (0.03002180081 + 2.540100125 sinx cosx) sin(3. x)

      + (-0.2040095464 + 1.523996800 sinx cosx) sin(5. x)

      + (-0.1703333837 - 0.8932621544 sinx cosx) sin(7. x)

      + (-0.02982150529 - 2.489260003 sinx cosx) sin(9. x)

      + (0.08202785276 - 1.796643686 sinx cosx) sin(11. x)

      + (0.09564846028 + 0.5477985513 sinx cosx) sin(13. x)

      + (0.02942331960 + 2.388597326 sinx cosx) sin(15. x)

      + (-0.04508855063 + 2.033330735 sinx cosx) sin(17. x)

      + (-0.06682310642 - 0.1913707567 sinx cosx) sin(19. x)

      + (-0.02883202008 - 2.240126858 sinx cosx) sin(21. x)

      + (0.02675490134 - 2.229320656 sinx cosx) sin(23. x)

      + (0.05081744906 - 0.1688873247 sinx cosx) sin(25. x)

      + (0.02805468571 + 2.046820234 sinx cosx) sin(27. x)

      + (-0.01558288316 + 2.380690709 sinx cosx) sin(29. x)

      + (-0.04018727876 + 0.5257651252 sinx cosx) sin(31. x)

      + (-0.02710059726 - 1.812546490 sinx cosx) sin(33. x)

      + (0.007982846823 - 2.484411222 sinx cosx) sin(35. x)

      + (0.03233081750 - 0.8721197336 sinx cosx) sin(37. x)

      + (0.02598110487 + 1.541994616 sinx cosx) sin(39. x)

plot(u,x=0..L);
Warning,  expecting only range variable x in expression  (1.117372821+1.220847109*sinx*cosx)*sin(x)+(.3002180081e-1+2.540100125*sinx*cosx)*sin(3.*x)+(-.2040095464+1.523996800*sinx*cosx)*sin(5.*x)+(-.1703333837-.8932621544*sinx*cosx)*sin(7.*x)+(-.2982150529e-1-2.489260003*sinx*cosx)*sin(9.*x)+(.8202785276e-1-1.796643686*sinx*cosx)*sin(11.*x)+(.9564846028e-1+.5477985513*sinx*cosx)*sin(13.*x)+(.2942331960e-1+2.388597326*sinx*cosx)*sin(15.*x)+(-.4508855063e-1+2.033330735*sinx*cosx)*sin(17.*x)+(-.6682310642e-1-.1913707567*sinx*cosx)*sin(19.*x)+(-.2883202008e-1-2.240126858*sinx*cosx)*sin(21.*x)+(.2675490134e-1-2.229320656*sinx*cosx)*sin(23.*x)+(.5081744906e-1-.1688873247*sinx*cosx)*sin(25.*x)+(.2805468571e-1+2.046820234*sinx*cosx)*sin(27.*x)+(-.1558288316e-1+2.380690709*sinx*cosx)*sin(29.*x)+(-.4018727876e-1+.5257651252*sinx*cosx)*sin(31.*x)+(-.2710059726e-1-1.812546490*sinx*cosx)*sin(33.*x)+(.7982846823e-2-2.484411222*sinx*cosx)*sin(35.*x)+(.3233081750e-1-.8721197336*sinx*cosx)*sin(37.*x)+(.2598110487e-1+1.541994616*sinx*cosx)*sin(39.*x)  to be plotted but found names [cosx, sinx]