\(=\frac{2}{20}+\frac{2}{30}+...+\frac{2}{420}\)
\(=\frac{2}{4.5}+\frac{2}{5.6}+...+\frac{2}{20.21}\)
\(=2\left(\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+...+\frac{1}{20}-\frac{1}{21}\right)\)
\(=2\left(\frac{1}{4}-\frac{1}{21}\right)\)
\(=2\times\frac{17}{84}\)
\(=\frac{17}{72}\)
B=\(\frac{1}{10}+\frac{1}{15}+\frac{1}{21}+...+\frac{1}{120}\)
tính:
\(\frac{1}{10}+\frac{1}{15}+\frac{1}{21}+.........+\frac{1}{120}\)
Tính nhanh: N = \(\frac{1}{10}+\frac{1}{15}+\frac{1}{21}+...+\frac{1}{120}\)
\(\frac{1}{10}+\frac{1}{15}+\frac{1}{21}+...+\frac{1}{120}\)
CỨU
\(\frac{x}{2008}-\frac{1}{10}-\frac{1}{15}-\frac{1}{21}...\frac{1}{120}=\frac{5}{8}\)Tìm x
TINH\(C=\frac{x}{2008}-\frac{1}{10}-\frac{1}{15}-\frac{1}{21}-...-\frac{1}{120}=\frac{5}{8}\)
tinhn
\(\frac{1}{10}-\frac{1}{15}-\frac{1}{21}-...-\frac{1}{120}\)
P=\(\frac{1}{10}+\frac{1}{15}+\frac{1}{21}+..............+\frac{1}{120}\)
tìm x :
\(\frac{x}{2008}-\frac{1}{10}-\frac{1}{15}-\frac{1}{21}-....-\frac{1}{120}=\frac{5}{8}\)
