Question

найти предел (1/(2*4)+1/(4*6)+...+1/(2n*(2n+2))) СРОЧНО!

Answer 1

$\displaystyle \frac{1}{2\cdot4}+ \frac{1}{4\cdot6}+...+\frac{1}{2n \cdot(2n+2)}= \frac{1}{4} \bigg( \frac{1}{1\cdot2}+\frac{1}{2\cdot3}+...+\frac{1}{n\cdot(n+1)}\bigg)=\\\\ = \frac{1}{4} \bigg(\frac{2-1}{1\cdot2}+\frac{3-2}{2\cdot3}+...+\frac{n+1-n}{n\cdot(n+1)}\bigg)=\\ \\ \\ = \frac{1}{4} \bigg(1- \frac{1}{2} + \frac{1}{2} -\frac{1}{3}+...+\frac{1}{n} -\frac{1}{n+1}\bigg)= \frac{1}{4} \bigg(1-\frac{1}{n+1}\bigg)= \frac{1}{4} ,~~n\to\infty$