Question

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

Answer 1

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

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

\(A=\frac{3-2}{2.3}+\frac{4-3}{3.4}+\frac{5-4}{4.5}+\frac{6-5}{5.6}+\frac{7-6}{6.7}+\frac{8-7}{7.8}+\frac{9-8}{8.9}\)

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

\(A=\frac{1}{2}-\frac{1}{9}\)

\(A=\frac{7}{18}\)

Vậy \(A=\frac{7}{18}\)

Answer 2

A = 1/2.3 + 1/3.4 + .....  +1/8.9

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

    = 1/2 - 1/9

    = 7/18

Tk mk nha

Answer 3

A = 1/2.3 + 1/3.4 + .....  +1/8.9

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

    = 1/2 - 1/9

    = 7/18

 

Answer 4

\(A=\frac{1}{2}.\frac{1}{3}+\frac{1}{3}\cdot\frac{1}{4}+...+\frac{1}{8}\cdot\frac{1}{9}\)

\(A=\frac{1}{2.3}+\frac{1}{3.4}+....+\frac{1}{8\cdot9}\)

\(A=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{8}-\frac{1}{9}\)

\(A=\frac{1}{2}-\frac{1}{9}\)

\(A=\frac{7}{18}\)

Answer 5

A = 1/2 . 1/3 + 1/3 . 1/4 + 1/4 . 1/5 + ...+ 1/7 . 1/8 + 1/8 . 1/9

A =    1/2.3 + 1/3.4 + 1/4.5 + ...+ 1/7.8 + 1/8.9 

A = 1/2 - 1/3 + 1/3 - 1/4 + ...+ 1/7 - 1/8 + 1/8 - 1/ 9

A = 1/2 - 1/9

A = 9/18 - 2/18

A = 7/18 

Vậy A = 7/18 

Chúc bạn học giỏi !!! ^_^ 

Answer 6

=7/18

Answer 7

A=(1/2-1/2+1/3-1/3+1/4-1/4+1/5-1/5+1/6-1/6+1/7-1/7+1/8-1/8)+1/9

A=(1/2-1/8)+1/9

A=3/8+1/9

A=35/72