Question

bài 4 : cmr :

c) C = \(\dfrac{1}{2}.\dfrac{3}{4}.\dfrac{5}{6}....\dfrac{9999}{10000}< \dfrac{1}{100}\)

Answer 1

Ta có: \(C=\dfrac{1}{2}\cdot\dfrac{3}{4}\cdot\dfrac{5}{6}...\dfrac{9999}{10000}\)

\(C=\dfrac{1\cdot3\cdot5...9999}{2\cdot4\cdot6...10000}\)

Gọi \(D=\dfrac{2}{3}\cdot\dfrac{4}{5}\cdot\dfrac{6}{7}...\dfrac{10000}{10001}\)

Mà \(\dfrac{1}{2}< \dfrac{2}{3};\dfrac{3}{4}< \dfrac{4}{5};\dfrac{5}{6}< \dfrac{6}{7};...;\dfrac{9999}{10000}< \dfrac{10000}{10001}\)

\(\Rightarrow\dfrac{1}{2}\cdot\dfrac{3}{4}\cdot\dfrac{5}{6}...\dfrac{9999}{10000}< \dfrac{2}{3}\cdot\dfrac{4}{5}\cdot\dfrac{6}{7}...\dfrac{10000}{10001}\)

\(\Rightarrow C< D\)

Ta lại có: \(C\cdot D=\left(\dfrac{1}{2}\cdot\dfrac{3}{4}\cdot\dfrac{5}{6}...\dfrac{9999}{10000}\right)\left(\dfrac{2}{3}\cdot\dfrac{4}{5}\cdot\dfrac{6}{7}...\dfrac{10000}{10001}\right)\)

\(\Rightarrow C\cdot D=\dfrac{1}{2}\cdot\dfrac{2}{3}\cdot\dfrac{3}{4}\cdot\dfrac{4}{5}\cdot\dfrac{5}{6}\cdot\dfrac{6}{7}...\dfrac{9999}{10000}\cdot\dfrac{10000}{10001}\)

\(\Rightarrow C\cdot D=\dfrac{1\cdot2\cdot3\cdot4\cdot5\cdot6...9999\cdot10000}{2\cdot3\cdot4\cdot5\cdot6\cdot7...10000\cdot10001}\)

\(\Rightarrow C\cdot D=\dfrac{1}{10001}\)

Mà \(C< D\)

\(\Rightarrow C\cdot C< C\cdot D\)

\(\Rightarrow C\cdot C< \dfrac{1}{10001}\)

\(\Rightarrow C< \dfrac{1}{10001}\)

Mà \(\dfrac{1}{10001}< \dfrac{1}{100}\)

\(\Rightarrow C< \dfrac{1}{100}\)

Vậy \(C< \dfrac{1}{100}\)

Answer 2

C = \(\dfrac{1}{2}\).\(\dfrac{3}{4}\).\(\dfrac{5}{6}\)....\(\dfrac{9999}{10000}\)

C < \(\dfrac{2}{3}.\dfrac{4}{5}.\dfrac{6}{7}....\dfrac{10000}{10001}\)

C² < \(\dfrac{1.\left(3.5.7...9999\right)}{\left(2.4.6...10000\right)}.\dfrac{\left(2.4.6...10000\right)}{\left(3.5.7...9999\right).10001}\)

C^{2 < \(\dfrac{1}{10001}\)}

C² < \(\left(\dfrac{1}{100}\right)^2\)

C < \(\dfrac{1}{100}\)

Vậy \(\dfrac{1}{2}.\dfrac{3}{4}.\dfrac{5}{6}....\dfrac{9999}{10000}< \dfrac{1}{100}\)

Chúc bạn học tốt !

Answer 3

ta có : 1/2 < 2/3

2/3 <3/4

......... 9999/10000 < 10000/10001 suy ra : A2 < 1/22/33/4*****9999/1000010000/10001 suy ra : A2 < 1/10001 < 1/10000= (1/100)2 suy ra A2 < (1/100)2 . Từ đó: A < 1/100 = 0,01

Answer 4

tks các pn Phạm_Tiến_Đức ; Nguyễn Lưu Vũ Quang &Giọt nước mắt nhẹ rơi tks các pn nhìu lắm . Các pn thật tốt bụng