Question

tìm x biết 1/2.1 +1/2.3+1/3.4 +......+1/x.(x+1) =2014/2015

Answer 1

Lời giải:

$\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+....+\frac{1}{x(x+1)}=\frac{2014}{2015}$

$\frac{2-1}{1.2}+\frac{3-2}{2.3}+\frac{4-3}{3.4}+...+\frac{(x+1)-x}{x(x+1)}=\frac{2014}{2015}$

$1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{x}-\frac{1}{x+1}=\frac{2014}{2015}$

$1-\frac{1}{x+1}=\frac{2014}{2015}$

$\frac{1}{x+1}=1-\frac{2014}{2015}=\frac{1}{2015}$
$\Rightarrow x+1=2015$

$\Rightarrow x=2014$