Help with Plotting Fourier Series Solutions to PDEs

When you define a solution ( for example u(x,t) ) in terms of the first few terms of a Fourier series, for example
u[x_,t_]:=Sum[c[n]Sin[n x]Exp[5 t],{n,1,25}]
and you want to plot this for a specific value of t, say t=2,
DO NOT use
Plot[u[x,2],{x,1,5}].
This will take a loooooooooooong time.
Instead, define the function separately and then plot. For example:
u2=u[x,2]
Plot[u2,{x,1,5}]
This forces Mathematica to "remember" the specific value of u[x,2] instead of having to recalculate it again. It may take a while to return u2, but that and the plot should take much less time than doing it the other way.

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