Question

tính

1/72 +1/56 + 1/42 + 1/30 + 1/20+ 1/12

Answer 1

1/72 + 1/56 + 1/42 + 1/30 + 1/20 + 1/12 

= 1/( 8 .9 ) + 1/ ( 7 . 8 ) + 1/ ( 6 . 7 ) + 1/ ( 5 . 6 ) + 1/ ( 4 . 5 ) + 1/( 3 . 4 )

=  1/3 - 1/4 + 1/5 -1/6 + 1/6 - 1/7 + 1/7 - 1/8 + 1/8 - 1/9 

=  1/3 - 1/9 

=  3/9 - 1/9 

= 2/9 

Chắc 100%

Answer 2

Ta có : 

\(\frac{1}{72}+\frac{1}{56}+\frac{1}{42}+\frac{1}{30}+\frac{1}{20}+\frac{1}{12}\)

\(=\)\(\frac{1}{8.9}+\frac{1}{7.8}+\frac{1}{6.7}+\frac{1}{5.6}+\frac{1}{4.5}+\frac{1}{3.4}\)

\(=\)\(\frac{1}{8}-\frac{1}{9}+\frac{1}{7}-\frac{1}{8}+\frac{1}{6}-\frac{1}{7}+\frac{1}{5}-\frac{1}{6}+\frac{1}{4}-\frac{1}{5}+\frac{1}{3}-\frac{1}{4}\)

\(=\)\(-\frac{1}{9}+\frac{1}{3}\)

\(=\)\(\frac{-1}{9}+\frac{3}{9}\)

\(=\)\(\frac{2}{9}\)