1/4.5+1/5.6+.+1/x(x+1)=21/100
=>1/4-1/5+1/5-1/6+.......+1/x-1/(x+1)=21/100
=>1/4-1/(x+1)=21/100
=>1/(x+1)=1/4-21/100
=>1/(x+1)=1/25
=>x+1=25
=>x=24
vậy.......
(1/4 -1/5 + 1/5 -1/6 +... +1/x - 1/x+1) = 21/100
=> 1/4 -1/x+1= 21/100
=>1/x+1=1/2-21/100
=>1/x+1= 1/25
=>x+1=25
=>x=24
Chúc bn hok giỏi
\(\frac{1}{4\times5}+\frac{1}{5\times6}+....+\frac{1}{x\times\left(x+1\right)}=\frac{21}{100}\)
\(\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+...+\frac{1}{x}-\frac{1}{x+1}=\frac{21}{100}\)
\(\frac{1}{4}-\frac{1}{x+1}-\frac{21}{100}=0\)
\(\left(\frac{1}{4}-\frac{21}{100}\right)-\frac{1}{x+1}=0\)
\(\frac{1}{25}-\frac{1}{x+1}=0\)
\(\frac{x+1}{25\times\left(x+1\right)}-\frac{25}{25\times\left(x+1\right)}=0\)
\(\frac{x+1-25}{25\times\left(x+1\right)}=0\)
\(\frac{x-24}{25\times\left(x+1\right)}=0\)
\(\Rightarrow x-24=0\)
\(x=24\)
Bài này mk giải theo cách lớp sáu cậu ạ
tk cho mk nhé -.-
