Gọi A = 2/11.13 +2/13.15+2/15.17 +...+2/53.55

=> A = 1/11-1/13+1/13-1/15+...+1/53-1/55

=> A = 1/11-1/55

=> A = 4/55

Đúng 100%

=1/11-1/13+1/13-1/15+1/15-1/17+...+1/53-1/55

=1/11-1/55

=5/55-1/55

=4/55

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bạn vào đây sẽ thấy bài giải mik vừa giải xong đó

cho mik đúng nha

Đặt \(A=\frac{2}{11.13}+\frac{2}{13.15}+\frac{2}{15.17}+\frac{2}{17.19}+\frac{2}{19.21}^{ }\)

\(A=\frac{1}{11}-\frac{1}{13}+\frac{1}{13}-\frac{1}{15}+\frac{1}{15}-\frac{1}{17}+\frac{1}{17}-\frac{1}{19}+\frac{1}{19}-\frac{1}{21}\)

\(A=\frac{1}{11}-\frac{1}{21}\)

\(A=\frac{10}{231}\)

Thay \(A=\frac{10}{231}\) vào ta có

\(\frac{10}{231}.462-x=19\)

\(20-x=19\)

\(x=20-19\)

\(x=1\)

boi vi\(\frac{10}{231}.462=20\) cho mik dung nha

=(1/11-1/13)+(1/13-1/15)+(1/15-1/17)+(1/17-1/19)+(1/19-1/21)*462-X=19

=(1/11-1/21)*462-X=19

=10/231*462-X=19

=20-X=19

=X=20-19

=X=1

lop nam cung lam duoc

Đề là tìm x hả bạn!!

     x - (2/11.13 + 2/13.15 + 2/15.17 +...+ 2/53.55) = 3/11 

=> x - (1/11 - 1/13 + 1/13 - 1/15 + 1/15 - 1/17 +...+ 1/53 - 1/55) = 3/11

=> x - (1/11 - 1/55) = 3/11

=> x - (5/55 - 1/55) = 3/11

=> x - 4/55 = 15/55

=> x = 15/55 + 4/55

=> x = 19/55 

\(\left(\frac{2}{11.13}+\frac{2}{13.15}+\frac{2}{15.17}+\frac{2}{17.19}+\frac{2}{19.21}\right)\) ) . 462 - x = 19

\(\left(\frac{1}{11}-\frac{1}{13}+\frac{1}{13}-\frac{1}{15}+\frac{1}{15}-\frac{1}{17}+\frac{1}{17}-\frac{1}{19}+\frac{1}{19}-\frac{1}{21}\right)\) . 462 - x  = 19

\(\left(\frac{1}{11}-\frac{1}{21}\right)\) . 462 - x = 19 

... Chúc bạn học tốt !

\(\left(\frac{2}{11\cdot13}+\frac{2}{13\cdot15}+\frac{2}{15\cdot17}+\frac{2}{17\cdot19}+\frac{2}{19\cdot21}\right)\)\(\cdot462-x=19\)

\(\left(\frac{1}{11\cdot13}+\frac{1}{13\cdot15}+\frac{1}{15\cdot17}+\frac{1}{17\cdot19}+\frac{1}{19\cdot21}\right)\)\(\cdot462-x=19\)

\(\left(\frac{1}{11}-\frac{1}{13}+\frac{1}{13}-\frac{1}{15}+\frac{1}{15}-\frac{1}{17}+\frac{1}{17}-\frac{1}{19}+\frac{1}{19}-\frac{1}{21}\right)\)\(\cdot462-x=19\)

\(\left(\frac{1}{11}-\frac{1}{21}\right)\cdot462-x=19\)

\(\frac{10}{231}\cdot462-x=19\)

\(20-x=19\)

\(x=20-19\)

\(x=1\)

cảm ơn mọi người

mình đoán là 2

\(\frac{2}{11.13}+\frac{2}{13.15}+\frac{2}{15.17}+...+\frac{2}{97.99}\)

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

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

\(=\frac{9}{99}-\frac{1}{99}=\frac{8}{99}\)

8/99 nha

Nhớ tk mình

Chúc bạn học tốt

^.^