Question

[le lan huong]

1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90

trình bày ra hẳn nhé thankyou 

Answer 1

[An Hoà]

\(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}+\frac{1}{90}\)

=\(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+\frac{1}{8.9}+\frac{1}{9.10}\)

=\(\frac{1}{1}-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3.}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}+\frac{1}{9}-\frac{1}{10}\)

=\(\frac{1}{1}-\frac{1}{10}\)

=\(\frac{9}{10}\)

Answer 2

[Sherlockichi Kazukosho]

1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90

= 1 / 1.2 + 1 / 2.3 + ` 1 / 3. 4 + 1 / 4 .5 + 1/5 .6 + 1/6 . 7 + 1 / 7.8 + 1 / 8.9 + 1 /9.10

= 1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + .... + 1/8 - 1/9 + 1/9 - 1/10

= 1/1 - 1/10

= 9/10