10 x 20 - 4 x 20

= 200 - 80

=120

Đúng thì k mk nha!^-^

Ta có : \(10\times20-4\times20\)

\(=\left(10-4\right)\times20\) 

\(=6\times20\)

\(=120\)

Có thể là:

      10 x 20 - 4 x 20

=20 x (20-4)

=20 x 6

=120

200

= 200 

200 ezy

B = 5^2/ 10.15 + 5^2/ 15.20 +....+ 5^2/190.195 + 5^2/195.200

B= 25/10.15 + 25/15.20 +........+ 25/190.105 + 25/195.200

B= 25( 1/10.15 + 1/15.20+....+ 1/190.195+ 195/195.200

B= 5( 1/10-1/15 + 1/15-1/20+...+1/195- 1/200)

B= 5. ( 1/10 - 1/200)

B= 5.19/200

B= 19/40

Vậy..........

cảm ơn Duki ạ nhưng cho mik hỏi tại sao 25(1/10.15...) lại =5(...) còn lại mik đã hiểu mong bạn trả lời,mik cảm ơn.

\(B=\frac{5^2}{10.15}+\frac{5^2}{15.20}+...+\frac{5^2}{195.200}\)

\(< =>B=5\left(\frac{5}{10.15}+\frac{5}{15.20}+...+\frac{5}{195.200}\right)\)

\(< =>B=5\left(\frac{1}{10}-\frac{1}{15}+\frac{1}{15}-...+\frac{1}{195}-\frac{1}{200}\right)\)

\(< =>B=5\left(\frac{19}{200}\right)=\frac{19}{40}\)

N = 1/1x5 + 1/5x10 + 1/10x15 + 1/15x20 + .....+1/2005 x 2010

N = 1 - 1/5 +1/5-1/5+1/10-1/15+1/5-1/20+.....+1/2005-1/2010

N = 1 - 1/2010

N = 2009/2010

Ta có:

\(N=\frac{1}{1x5}+\frac{1}{5x10}+\frac{1}{10x15}...+\frac{1}{2005x2010}\)

\(\Rightarrow Nx5=\left(\frac{1}{1x5}+\frac{1}{5x10}+\frac{1}{10x15}...+\frac{1}{2005x2010}\right)x5\)

\(=\frac{5}{1x5}+\frac{5}{5x10}+\frac{5}{10x15}...+\frac{5}{2005x2010}\)

\(=1-\frac{1}{5}+\frac{1}{5}-\frac{1}{10}+\frac{1}{10}-\frac{1}{15}+...+\frac{1}{2005}-\frac{1}{2010}\)

\(=1-\frac{1}{2010}\)

\(=\frac{2009}{2010}\)

\(\Rightarrow N=\frac{2009}{2010}:5=\frac{2009}{2010}x\frac{1}{5}=\frac{2009}{10050}\)

Bạn thuỳ dung ơi cho hỏi sao 5/(1x5) = 1 - 1/5 vậy ???

A = 1/5×10 + 1/10×15 + ... + 1/100×105

A = 1/5×5 × (1/1×2 + 1/2×3 + ... + 1/20×21)

A = 1/25 × (1 - 1/2 + 1/2 - 1/3 + ... + 1/20 - 1/21)

A = 1/25 × (1 - 1/21)

A = 1/25 × 20/21

A = 4/105

A = \(\frac{1}{5.10}+\frac{1}{10.15}+...+\frac{1}{100.105}\)

A = \(\frac{1}{5}-\frac{1}{10}+\frac{1}{10}-\frac{1}{15}+...\frac{1}{100}-\frac{1}{105}\)

A = \(\frac{1}{5}-\frac{1}{105}\)

A = \(\frac{20}{105}\)

K MK NHA

\(A=\frac{1}{5.10}+\frac{1}{10.15}+....+\frac{1}{100.105}=\frac{1}{5}\left(\frac{1}{5}-\frac{1}{10}+\frac{1}{10}-\frac{1}{15}+....+\frac{1}{100}-\frac{1}{105}\right)\)

\(\Rightarrow A=\frac{1}{5}\left(\frac{1}{5}-\frac{1}{105}\right)=\frac{1}{5}.\frac{4}{21}=\frac{4}{105}\)

(Dấu chấm . là dấu nhân nha bạn)

\(\frac{2}{1.3}+\frac{3}{3.6}+\frac{4}{6.10}+\frac{5}{10.15}+\frac{6}{15.21}\)

= \(1-\frac{1}{3}+\frac{1}{3}-\frac{1}{6}+\frac{1}{6}-\frac{1}{10}+\frac{1}{10}-\frac{1}{15}+\frac{1}{15}-\frac{1}{21}\)

= \(1-\frac{1}{21}\)

= \(\frac{20}{21}\)

Ủng hộ mk nha !!! ^_^

\(N=\frac{1}{1.5}+\frac{1}{5.10}+...+\frac{1}{2005.2010}=\frac{1}{5}\left(\frac{5}{1.5}+\frac{5}{5.10}+...+\frac{5}{2005.2010}\right)=\frac{1}{5}\left(\frac{1}{1}-\frac{1}{5}+...+\frac{1}{2005}-\frac{1}{2010}\right)\)

\(N=\frac{1}{5}.\frac{2009}{2010}=\frac{2009}{2010}\)

Bài bạn Mạnh Chưa đúng. bạn  Hạnh kiểm tra lại nhé

\(N=\frac{1}{1.5}+\frac{1}{5.10}+....+\frac{1}{2005.2010}\)

\(N=5.\left(\frac{1}{1}-\frac{1}{5}+\frac{1}{5}-\frac{1}{10}+....+\frac{1}{2005}-\frac{1}{2010}\right)\)

\(N=5.\left(\frac{1}{1}-\frac{1}{2010}\right)\)

\(N=5.\frac{2009}{2010}\)

\(N=\frac{2009}{402}\)