Quy luật là :
20=4x5;30=5x6;42=6x7......240=15x16
Còn lại mình không biết . Để mình coi
\(A=\frac{2}{20}+\frac{2}{30}+\frac{2}{42}+...+\frac{2}{240}\)
\(=\frac{2}{4.5}+\frac{2}{5.6}+\frac{2}{6.7}+...+\frac{2}{15.16}\)
\(=2\left(\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+...+\frac{1}{15}-\frac{1}{16}\right)\)
\(=2\left(\frac{1}{4}-\frac{1}{16}\right)=2\left(\frac{4-1}{16}\right)=2.\frac{3}{16}=\frac{3}{8}\)
(2/4-2/5+2/5-2/6.....+2/15-2/16)=
2/4-2/16=3/8
A=\(\frac{2}{20}+\frac{2}{30}+...+\frac{2}{240}\)
A= \(\frac{2}{4.5}+\frac{2}{5.6}+....+\frac{2}{15.16}\)
A=\(2.\left(\frac{1}{4.5}+\frac{1}{5.6}+...+\frac{1}{15.16}\right)\)
A=\(2.\left(\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+...+\frac{1}{15}-\frac{1}{16}\right)\)
A= \(2\left(\frac{1}{4}-\frac{1}{16}\right)\)
A= \(2.\frac{3}{16}\)
A= \(\frac{6}{16}\)
A=\(\frac{3}{8}\)
