1.
\(\left(\dfrac{1}{1.2.3}+\dfrac{1}{2.3.4}+--...+\dfrac{1}{8.9.10}\right)x=\dfrac{23}{45}\)
\(\Leftrightarrow\left[\dfrac{1}{2}\left(\dfrac{2}{1.2.3}+\dfrac{2}{3.4.5}+=+\dfrac{2}{8.9.10}\right)\right]x=\dfrac{23}{45}\)
\(\Leftrightarrow\left[\dfrac{1}{2}\left(\dfrac{1}{1.2}-\dfrac{1}{2.3}+\dfrac{1}{2.3}-\dfrac{1}{3.4}+--+\dfrac{1}{8.9}-\dfrac{1}{9.10}\right)\right]x=\dfrac{23}{45}\)
\(\Leftrightarrow\left[\dfrac{1}{2}\left(\dfrac{1}{1.2}-\dfrac{1}{8.9}\right)\right]x=\dfrac{23}{45}\)
\(\Leftrightarrow\left[\dfrac{1}{2}.\dfrac{22}{45}\right]x=\dfrac{23}{45}\)
\(\Leftrightarrow\dfrac{11}{45}.x=\dfrac{23}{45}\)
\(\Leftrightarrow x=\dfrac{23}{11}\)
Vậy \(x=\dfrac{23}{11}\) là giá trị cần tìm
2.
\(\dfrac{1}{3}+\dfrac{1}{6}+\dfrac{1}{10}+--.+\dfrac{1}{x\left(x+1\right):2}=\dfrac{1998}{2000}\)
\(\Leftrightarrow\dfrac{2}{6}+\dfrac{2}{12}+\dfrac{2}{20}+=...+\dfrac{2}{x\left(x+1\right)}=\dfrac{1998}{2000}\)
\(\Leftrightarrow\dfrac{2}{2.3}+\dfrac{2}{3.4}+\dfrac{2}{4.5}+--...+\dfrac{2}{x\left(x+1\right)}=\dfrac{1998}{2000}\)
\(\Leftrightarrow2\left(\dfrac{1}{2.3}+\dfrac{1}{3.4}+\dfrac{1}{5.6}+--.+\dfrac{1}{x\left(x+1\right)}\right)=\dfrac{1998}{2000}\)
\(\Leftrightarrow2\left(\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+--.+\dfrac{1}{x}-\dfrac{1}{x+1}\right)=\dfrac{1998}{2000}\)
\(\Leftrightarrow2\left(\dfrac{1}{2}-\dfrac{1}{x+1}\right)=\dfrac{1998}{2000}\)
\(\Leftrightarrow\dfrac{1}{2}-\dfrac{1}{x+1}=\dfrac{999}{2000}\)
\(\Leftrightarrow\dfrac{1}{x+1}=\dfrac{1}{2000}\)
\(\Leftrightarrow x+1=2000\)
\(\Leftrightarrow x=1999\)
Vậy \(x=1999\) là giá trị cần tìm
