Symsum undefined function error

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I am trying to plot an equation with an infinite series using symsum.

I get this error when I try to plot it.

>> plot(x,.9-symsum((2./(n*3.14159)).*(sin(.1*n*3.14159))*(exp(-(n.^2)*(3.14159.^2)*x)),n,1,Inf));

Undefined function 'symsum' for input arguments of type 'double'.

Based on previous questions I thought it might be a problem of having the toolbox but that is in the right directory. Maybe I can't use symsum with my argument type, but is there another function I can use? Sum also didn't work (I got: Error using sum Dimension argument must be a positive integer scalar within indexing range.)