La Serie de Fourier de 

f(x) = \left| x \right| , \:\:si \:\:-1≤x≤1,\:\:\:\:y\:\:\:\:f(x+2)=f(x)

\left|x  \right| ∼\frac{1}{2}-\frac{4}{π^2}\sum_{n=1}^{∞}{\frac{cos(2n-1)πx}{n^2} }

k = 1 (slider from 1 to 50, step 1)
seno = Sequence(cos(((((2 * n) - 1) * π) * x)) / ((2 * n) - 1)^(2), n, 1, k)
g(x) = If[-1 ≤ x ≤ 1, abs(x)]
h(x) = g(x - 2)
p(x) = g(x - 4)
q(x) = g(x - 6)
r(x) = g(x + 2)
s(x) = g(x + 4)
t(x) = g(x + 6)
f(x) = 1 / 2 - (4 / π^(2) * Sum[seno])