\frac{\pi }{4}=\sum _{n=0}^{\infty } \frac{(-1)^n}{2n+1}=1-\frac{1}{3}+\frac{1}{5}-\frac{1}{7}+\frac{1}{9}-\frac{1}{11}+\text{...}+\frac{(-1)^n}{2n+1}