A=\(\frac{4}{11}-\frac{4}{13}+\frac{4}{13}-\frac{4}{15}+...+\frac{4}{99}-\frac{4}{101}\)

\(A=4\left(\frac{1}{11}-\frac{1}{13}+\frac{1}{13}-\frac{1}{15}+...+\frac{1}{99}-\frac{1}{101}\right)\)

\(A=4.\left(\frac{1}{11}-\frac{1}{101}\right)\)

A=4. 90/1111=360/1111

Co dung khong vay Cuong Lucha DT

Câu1:                     750

Câu2:                     \(\approx\)5240,322

hơi tốn giấy đó bạn 

bạn đừng có làm hao giấy thế chứ

\(A=13.15+15.17+17.19+...+99.101\)

\(\Rightarrow6A=13.15.6+15.17.6+17.19.6+...+99.101.6\)

\(\Rightarrow6A=13.15.\left(17-11\right)+15.17.\left(19-13\right)+17.19.\left(21-15\right)+...+99.101.\left(103-97\right)\)

\(\Rightarrow6A=\left(13.15.17-11.13.15\right)+\left(15.17.19-13.15.17\right)+\left(17.19.21-15.17.19\right)+...+\left(99.101.103-97.99.101\right)\)

\(\Rightarrow6A=99.101.103-11.13.15\)

\(\Rightarrow6A=1027752\)

\(\Rightarrow A=171292\)

\(\dfrac{4}{9\cdot11}+\dfrac{4}{11\cdot13}+...+\dfrac{4}{97\cdot99}\)

\(=2\left(\dfrac{2}{9\cdot11}+\dfrac{2}{11\cdot13}+...+\dfrac{2}{97\cdot99}\right)\)

\(=2\left(\dfrac{1}{9}-\dfrac{1}{11}+\dfrac{1}{11}-\dfrac{1}{13}+...+\dfrac{1}{97}-\dfrac{1}{99}\right)\)

\(=2\cdot\left(\dfrac{1}{9}-\dfrac{1}{99}\right)\)

\(=2\cdot\dfrac{10}{99}=\dfrac{20}{99}\)

\(x-\frac{20}{11.13}-\frac{20}{13.15}-\frac{20}{15.17}-...-\frac{20}{53.55}=\frac{3}{11}\)

\(\Rightarrow x-\left(\frac{20}{11.13}+\frac{20}{13.15}+\frac{20}{15.17}+...+\frac{20}{53.55}\right)=\frac{3}{11}\)

Đặt S = \(\frac{20}{11.13}+\frac{20}{13.15}+\frac{20}{15.17}+....+\frac{20}{53.55}\)

\(=10\left(\frac{2}{11.13}+\frac{2}{13.15}+\frac{2}{15.17}+...+\frac{2}{53.55}\right)\)

\(=10\left(\frac{1}{11}-\frac{1}{13}+\frac{1}{13}-\frac{1}{15}+\frac{1}{15}-\frac{1}{17}+...+\frac{1}{53}-\frac{1}{55}\right)\)

\(=10\left(\frac{1}{11}-\frac{1}{55}\right)\)

\(=10\cdot\frac{4}{55}=\frac{8}{11}\)

Thay A vào đề bài,ta được:

\(x-\frac{8}{11}=\frac{3}{11}\)

\(x=\frac{3}{11}+\frac{8}{11}\)

\(x=1\)

Đổi thay A thành thay S nha bạn

2/11x13+2/13x15+2/15x17+...+2/97x99

=1/11-1/13+1/13-1/15+1/15-1/17+...+1/97-1/99

=1/11-1/99

=8/99

thank you 

= 2/11.13 + 2/13.15 + ... + 2/97.99

= 2/11 .2/13 + 2/13 . 2/15 + ... + 2/97 . 2/97

= 2/11 + 2/97

= ......tu tinh

=2/11+2/13-2/13+2/15-2/15+...-2/97+2/98

=2/11+(2/13-2/13+2/15-2/15+...-2/97+2/99)

=2/11+2/99

=20/99

\(\frac{2}{11}\times13+\frac{2}{13}\times15+\frac{2}{15}\times17+........+\frac{2}{97}\times99\)

\(A\times2=\frac{2}{11\times13}+\frac{2}{13\times15}+\frac{2}{15\times17}+........+\frac{2}{97\times99}\)

\(A\times2=\frac{1}{11}-\frac{1}{13}+\frac{1}{13}-\frac{1}{15}+\frac{1}{15}-\frac{1}{17}+........+\frac{1}{97}-\frac{1}{99}\)

\(A=\frac{1}{11}-\frac{1}{99}\)

\(A=\frac{8}{99}\)

mik ko bt là có đúng hay ko nhưng đúng thì các bạn cho 1 t i c k nha