La Serie de Fourier de 

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

\sum_{n=1}^{∞}{ \frac{1}{n^2}}=\frac{π^2}{6}

x^2∼\frac{π^2}{3}+4\sum_{n=1}^{∞}{(-1)^n\frac{cos(nx)}{n^2} }

k = 1 (slider from 1 to 100, step 1)
seno = Sequence(((-1)^(n) * cos((n * x)) / n^(2)), n, 1, k)
g(x) = If[(-π)  <  x  <  π, x^(2)]
h(x) = If[π  <  x  <  (3 * π), (x - (2 * π))^(2)]
p(x) = If[(3 * π)  <  x  <  (5 * π), (x - (4 * π))^(2)]
q(x) = If[(5 * π)  <  x  <  (7 * π), (x - (6 * π))^(2)]
r(x) = If[(-3 * π)  <  x  <  (-π), (x + (2 * π))^(2)]
s(x) = If[(-5 * π)  <  x  <  (-3 * π), (x + (4 * π))^(2)]
t(x) = If[(-7 * π)  <  x  <  (-5 * π), (x + (6 * π))^(2)]
f(x) = π^(2) / 3 + (4 * Sum[seno])