Question

Tính:  1/15 + 1/35 +1/63 +1/99 + 1/143

Answer 1

Gọi dãy là A ta có :

A = 1/3.5 + 1/5.7 + 1/7.9 + 1/9.11 + 1/11.13

A = 1/2 . ( 2/3.5 + 2/5.7 + 2/7.9 + 2/9.11 + 2/11.13 )

A = 1/2 . ( 1/3 - 1/5 + 1/5 - 1/7 + 1/7 - 1/9 + 1/9 - 1/11 + 1/11 - 1/13 )

A = 1/2 . ( 1/3 - 1/13 )

A = 1/2 . 10/39

A = 5/39

Answer 2

\(\frac{1}{15}+\frac{1}{35}+\frac{1}{63}+\frac{1}{99}+\frac{1}{143}\)

=\(\frac{1}{2}\left(\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}+\frac{2}{9.11}+\frac{2}{11.13}\right)\)

=\(\frac{1}{2}\left(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+\frac{1}{9}-\frac{1}{11}+\frac{1}{11}-\frac{1}{13}\right)\)

=\(\frac{1}{2}\left(\frac{1}{3}-\frac{1}{13}\right)\)

=\(\frac{1}{2}.\frac{10}{39}\)

=\(\frac{5}{39}\)

Answer 3

ai có thể giải theo cách cấp 1 ko