Question

\(\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+...+\frac{1}{990}\)

giúp mik nhé!! 

Answer 1

\(\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+...+\frac{1}{990}\)

\(=\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+...+\frac{1}{99.100}\)

\(=\frac{1}{4}-\frac{1}{100}\)

\(=\frac{6}{25}\)

Answer 2

1-1/990=989/990

em la hoc sinh lop 4 

Answer 3

\(\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+.....+\frac{1}{990}\)

=\(\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+......+\frac{1}{99.100}\)

=\(\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+......+\frac{1}{99}-\frac{1}{100}\)(Phương pháp khử )

=\(\frac{1}{4}-\frac{1}{100}=\frac{100}{400}-\frac{4}{400}=\frac{96}{400}\)=\(\frac{6}{25}\)

Answer 4

ê ê mấy you sai rồi 990\(\ne\)99*100 mà
 

Answer 5

\(\frac{1}{99}\)- \(\frac{1}{100}\)= \(\frac{1}{9900}\) mà