\(1-\frac{1}{2}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{19}-\frac{1}{20}\)\(=\left(1+\frac{1}{3}+\frac{1}{5}+...+\frac{1}{19}\right)\)\(-\left(\frac{1}{2}+\frac{1}{4}+\frac{1}{6}+...+\frac{1}{20}\right)\)

\(=\left(1+\frac{1}{2}+\frac{1}{3}+...+\frac{1}{19}+\frac{1}{20}\right)\)\(-2\left(\frac{1}{2}+\frac{1}{4}+\frac{1}{6}+...+\frac{1}{20}\right)\)

\(=\left(1+\frac{1}{2}+\frac{1}{3}+...+\frac{1}{20}\right)\)\(-\left(1+\frac{1}{2}+\frac{1}{3}+...+\frac{1}{10}\right)\)

\(=\frac{1}{11}+\frac{1}{12}+\frac{1}{13}+...+\frac{1}{20}\)

a, 13/19

b, 5

c, 31/48

d, 6/7

mình cần cách giải

Mình cung song ngư nè

Bạn nhớ đăng nhiều thứ về cung song ngư

mk cung bo cap ( thien yet , than nong , ho cap )

Mk thuộc cung sư tử nhé

Mình thuộc cung Xử Nữ nè, có ai cung giống mình hông!

mik cung ma kết

1 - 1/2 + 1/3 - 1/4 + 1/5 - 1/6 + ...+ 1/19 - 1/20 

= ( 1 + 1/3 + 1/5 + ...+ 1/19 ) - ( 1/2 + 1/4 + ...+ 1/20 ) 

= ( 1 + 1/2 + 1/3 + 1/4 + ...+ 1/19 + 1/20 ) - 2 . ( 1/2 + 1/4 + ...+ 1/20 ) 

= ( 1 + 1/2 + 1/3 + ...+ 1/20 ) - ( 1 + 1/2 + ... + 1/10 ) 

=    1/11 + 1/12 + 1/13 + ...+ 1/20 ( Đpcm ) 
TK mk nha !!! 

\(1-\frac{1}{2}+\frac{1}{3}-\frac{1}{4}+.....+\frac{1}{19}-\frac{1}{20}\)

\(=\left(1+\frac{1}{3}+...+\frac{1}{19}\right)-\left(\frac{1}{2}+\frac{1}{4}+...+\frac{1}{20}\right)\)

\(=1+\frac{1}{2}+\frac{1}{3}+\frac{1}{4}+...+\frac{1}{19}+\frac{1}{20}-2\left(\frac{1}{2}+\frac{1}{4}+...+\frac{1}{20}\right)\)

\(=1+\frac{1}{2}+\frac{1}{3}+\frac{1}{4}+...+\frac{1}{20}-1+\frac{1}{2}+....+\frac{1}{10}\)

\(=\frac{1}{11}+\frac{1}{12}+...+\frac{1}{20}\left(đpcm\right)\)

\(1-\frac{1}{2}+\frac{1}{3}-\frac{1}{4}+\frac{1}{5}-\frac{1}{6}+.....+\frac{1}{19}-\frac{1}{20}\)

= \(\left(1+\frac{1}{3}+\frac{1}{5}+.........+\frac{1}{19}\right)-\left(\frac{1}{2}+\frac{1}{4}+\frac{1}{6}+.........+\frac{1}{20}\right)\)

= \(1+\frac{1}{2}+\frac{1}{3}+\frac{1}{4}+......+\frac{1}{19}+\frac{1}{20}-2\left(\frac{1}{2}+\frac{1}{4}+.......+\frac{1}{20}\right)\)

= \(1+\frac{1}{2}+\frac{1}{3}+\frac{1}{4}+..........+\frac{1}{19}+\frac{1}{20}+1+\frac{1}{2}+.............+\frac{1}{20}\)

= \(\frac{1}{11}+\frac{1}{12}+\frac{1}{13}+.........+\frac{1}{20}\)

Vậy biểu thức \(1-\frac{1}{2}+\frac{1}{3}-\frac{1}{4}+\frac{1}{5}-\frac{1}{6}+.........+\frac{1}{19}-\frac{1}{20}=\frac{1}{11}+\frac{1}{12}+\frac{1}{13}+.......+\frac{1}{20}\)( đpcm)

cái này tính cái gì thế

ko hiểu