La Serie Alternante para 

π

\sum_{n=1}^{∞ }{ \frac{4(-1)^{n+1}}{2n-1}}=\pi

k = 1 (slider from 1 to 7201, step 1)
alterna = Sequence(((4 * (-1)^(n + 1)) * 1 / ((2 * n) - 1)), n, 1, k)
a = Sum(alterna)
texto1: "\sum_{n=1}^{" + (LaTeX[k]) + "}{ \frac{4(-1)^{n+1}}{2n-1}}=" + (LaTeX[a]) + ""