Kreiszahl pi

Leibniz Reihe beschreibt genau $\pi$ /4 =1-1/3+1/5-1/7+1/9

$\sum _{k=0}^{\infty }{\frac {(-1)^{k}}{2k+1}}=1-{\frac {1}{3}}+{\frac {1}{5}}-{\frac {1}{7}}+{\frac {1}{9}}-\dotsb ={\frac {\pi }{4}}$.

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  Gottfried Wilhelm Leibniz Portrait