La Serie de Fourier de 

f(x) = x, \:\:si \:\:-π≤x≤π,\:\:\:\:y\:\:\:\:f(x+2π)=f(x)

x∼2\sum_{n=1}^{∞}{ \frac{(-1)^{n+1}}{n}sin(nx)}

k = 3 (slider from 1 to 50, step 1)
Fourier = Sequence(((-1)^(n + 1) * sin((n * x)) / n), n, 1, k)
a(x) = (2 * Sum[Fourier])
f(x) = If[(-π)  <  x  <  π, x]
g(x) = If[π  <  x  <  (3 * π), x - (2 * π)]
h(x) = If[(3 * π)  <  x  <  (5 * π), x - (4 * π)]
p(x) = If[(-3 * π)  <  x  <  (-π), x + (2 * π)]
q(x) = If[(5 * π)  <  x  <  (7 * π), x - (6 * π)]
r(x) = If[(-5 * π)  <  x  <  (-3 * π), x + (4 * π)]
s(x) = If[(-7 * π)  <  x  <  (-5 * π), x + (6 * π)]