# Plot complex solution obtained using DSolve?

Posted 5 months ago
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 Hi, I need to plot this stuff, but unfortunately that doesn't work and I don't understand why. If someone could help me? diffeq3 = x3''[t] + 10*x3[t] + 0.5*(x3[t] - x4[t]) == 0 diffeq4 = x4''[t] + 10*x4[t] + 0.5*(x4[t] - x3[t]) == 0 sol1 := DSolve[{diffeq3, diffeq4, x3'[0] == 0, x4'[0] == 0, x3[0] == xo3, x4[0] == xo4}, {x3[t], x4[t]}, t]; posx3[t_, xo3_, xo4_] = x3[t] /. sol1; posx4[t_, xo3_, xo4_] = x4[t] /. sol1; Plot[{posx3[t, 1, -1], posx4[t, 1, -1]}, {t, 0, 40}] The problem is at the end of my file. Attachments:
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Posted 5 months ago
 Razvan,Your general solution to the differential equation has complex numbers which go to zero when you ask for solutions that are real. However, because of machine precision, you have some VERY small complex parts. For example, In[20]:= posx3[0, 1, -1] Out[20]= {1. + 1.38667*10^-32 I} You must use Chop[] to get rid of infinitesimal imaginary parts. For example: sol1 := Chop[DSolve[{diffeq3, diffeq4, x3'[0] == 0, x4'[0] == 0, x3[0] == xo3, x4[0] == xo4}, {x3[t], x4[t]}, t]]; will fix this by Choping the numbers when evaluating posx3, posx4 or you can Chop it later at Plot time.Regards,Neil
Posted 5 months ago
 ty mate
Posted 5 months ago
 See my adjustments.Tested on: \$Version (* "11.3.0 for Microsoft Windows (64-bit) (March 7, 2018)" *) (* "10.2.0 for Microsoft Windows (64-bit) (July 7, 2015)" *)  Attachments:
Posted 5 months ago
 Ty mate
Posted 5 months ago
 Thank u both