= 7/4.(4/1.5 + 4/5.9 + 4/9.13 + 4/13.17 + 4/17.21)

 = 7/4.(1-1/5+1/5-1/9+1/9-1/13+1/13-1/17+1/17-1/21)

 = 7/4.(1-1/21)

 = 7/4.20/21 = 5/3

Tk mk nha

Đặt biểu thức bằng A

4/7A=1-1/5+1/5-1/9+...+1/17_1/21

4/7A=1-1/21

4/7A=20/21

A=35/21=5/3

Dễ ấy  mà

\(C=\frac{2}{4}.\left(\frac{4}{1.5}+\frac{4}{5.9}+\frac{4}{9.13}+\frac{4}{13.17}+\frac{4}{17.21}\right)\)

\(=\frac{1}{2}.\left(1-\frac{1}{5}+\frac{1}{5}-\frac{1}{9}+....+\frac{1}{17}-\frac{1}{21}\right)\)

\(=\frac{1}{2}.\left(1-\frac{1}{21}\right)\)

\(=\frac{1}{2}.\frac{20}{21}\)

\(=\frac{10}{21}\)

\(=2\left(\frac{2}{1.3}+\frac{2}{3.5}+\frac{2}{5.7}+...+\frac{2}{19.21}\right)\)

=\(2\left(1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{19}-\frac{1}{21}\right)\)

=\(2\left(1-\frac{1}{21}\right)\)

=\(\frac{2.20}{21}=\frac{40}{21}\)

Áp dụng theo dạng toán số ai cập ta có:

4/1.5+4/5.9+4/9.13+4/13.17+4/17.21=1/1-1/5+1/5-1/9+1/9-1/13+1/13-1/17+1/17-1/21=1-1/21 < 1

Vậy tổng đó < 1

\(\left(\frac{1}{3.5}+\frac{1}{5.7}+\frac{1}{7.9}+....+\frac{1}{19.21}\right).x=\frac{9}{7}\)

\(\left(\frac{5-3}{3.5}+\frac{7-5}{5.7}+\frac{9-7}{7.9}+...+\frac{21-19}{19.21}\right).x=\frac{9}{7}\)

\(\left[\frac{1}{2}.\left(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+...+\frac{1}{19}-\frac{1}{21}\right)\right].x=\frac{9}{7}\)

 \(\left[\frac{1}{2}.\left(\frac{1}{3}-\frac{1}{21}\right)\right].x=\frac{9}{7}\)

 \(\left[\frac{1}{2}.\frac{2}{7}\right].x=\frac{9}{7}\)

\(\frac{1}{7}.x=\frac{9}{7}\)

\(\Rightarrow x=\frac{9}{7}\div\frac{1}{7}=9\)

\(\left(\frac{1}{3.5}+\frac{1}{5.7}+\frac{1}{7.9}+...+\frac{1}{19.21}\right)x=\frac{9}{7}\)

\(\Leftrightarrow\left(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{19}-\frac{1}{21}\right)x=\frac{9}{7}\)

\(\Leftrightarrow\left(\frac{1}{3}-\frac{1}{21}\right)x=\frac{9}{7}\)

\(\Leftrightarrow\frac{2}{7}x=\frac{9}{7}\)

\(\Leftrightarrow x=\frac{9}{2}\)

2 ( 1/3.5 + 1/5.7 + 1/7.9+......+ 1/19.21) . x = 2.9/7

 (2/3.5 + 2/5.7 + 2/7.9+......+ 2/19.21). x = 18/7

(1/3-1/5 + 1/5-1/7+ 1/7-1/9 +.....+ 1/19 - 1/21).x = 18/7

(1/3-1/21) . x= 18/7

2/7 x = 18/7

x= 18/7 . 7/2

x= 9

\(\frac{4}{1\cdot3\cdot5}+\frac{4}{3\cdot5\cdot7}+\frac{4}{5\cdot7\cdot9}+\frac{4}{7\cdot9\cdot11}+\frac{4}{9\cdot11\cdot13}\)

\(=\frac{1}{1.3}-\frac{1}{3.5}+\frac{1}{3.5}-\frac{1}{5.7}+...+\frac{1}{9.11}-\frac{1}{11.13}\)

\(=\frac{1}{1.3}-\frac{1}{11.13}\)

\(=\frac{1}{3}-\frac{1}{143}\)

\(=\frac{140}{429}\)

13/30 nhé bạn

Kêt quả bằng 13/30

ta có:(3+4)/4=3/(3*4)+4/(3*4) =1/4+1/3 chứng minh tương tự,cộng vế với vế, ta được kết quả là 13/30