Question

Giúp mk cần gấp nhé

Answer 1

Answer 2

\(3A=\frac{1}{20}-\frac{1}{23}+\frac{1}{23}-\frac{1}{26}+...+\frac{1}{77}-\frac{1}{80}\\ \Leftrightarrow3A=\frac{1}{20}-\frac{1}{80}=\frac{3}{80}\\ \Leftrightarrow A=\frac{1}{80}\)

Answer 3

\(A=\frac{1}{20.23}+\frac{1}{23.26}+...+\frac{1}{77.80}\)

=\(\frac{1}{3}\left(\frac{1}{20}-\frac{1}{23}+\frac{1}{23}-\frac{1}{26}+...+\frac{1}{77}-\frac{1}{80}\right)\)

=\(\frac{1}{3}\left(\frac{1}{20}-\frac{1}{80}\right)\)=\(\frac{1}{3}.\frac{3}{80}=\frac{1}{80}\)