What you should do is to put your code in the form to run any mathematica, in that should make it using the buttons which are in the bar that I show you,
I hope that my explanation is understandable.
The result should be something like the following
(1/5)*Integrate[Sin[Pi*x - 4 Pi]^2*Sin[n*Pi*x/10], {x, 4, 5}];
A[n_] := -((400 (Cos[(2 n \[Pi])/5] - Cos[(n \[Pi])/2]))/(
n (-400 + n^2) \[Pi]));
A[20] := 0;
u[x_, t_, w_] := Sum[A[n]*Cos[n*Pi*t/5]*Sin[n*Pi*x/10], {n, 1, w}];
And as you'll see images are obtained as the following
