Question

2/1.2+2/2.3+2/3.4+....+2/8.9+2/9.10

Answer 1

ta có 2/1.2+2/2.3+2/3.4+...+2/8.9+2/9.10

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

=2/1-2/10

=9/5

Answer 2

Đặt A = \(\frac{2}{1.2}+\frac{2}{2.3}+\frac{2}{3.4}+...+\frac{2}{8.9}+\frac{2}{9.10}\)

\(A\times2=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{8.9}+\frac{1}{9.10}\)

\(A\times2=\frac{1}{1}-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{9}-\frac{1}{10}\)

\(A\times2=\frac{1}{1}-\frac{1}{10}\)

\(A=\frac{9}{10}\times\frac{1}{2}=\frac{9}{20}\)

Answer 3

Trần Quỳnh Mai sai rồi