\(\dfrac{3x4x7}{3x10x8}=\dfrac{84}{240}\)

\(\dfrac{3\times4\times7}{3\times10\times8}=\)\(\dfrac{3\times4\times7}{3\times10\times2\times4}=\dfrac{3\times7}{10\times2}=\dfrac{21}{20}\)

= 21/20 nha

7/12

??

nhầm

\(\dfrac{3.4.7}{3.4.8.9}=\dfrac{7}{72}\)

\(\dfrac{3\times4\times7}{12\times8\times9}\)
\(=\dfrac{12\times7}{12\times8\times9}\)
\(=\dfrac{7}{72}\)

`(3xx4xx7)/(12xx8xx9)=(3xx4xx7)/(3xx4xx8xxx9)= 7/(8xx9)=7/72`

Đặt \(A=\dfrac{1}{1\cdot2\cdot3}+\dfrac{1}{2\cdot3\cdot4}+\dfrac{1}{3\cdot4\cdot5}+...+\dfrac{1}{98\cdot99\cdot100}\)

Ta có: \(A=\dfrac{1}{1\cdot2\cdot3}+\dfrac{1}{2\cdot3\cdot4}+\dfrac{1}{3\cdot4\cdot5}+...+\dfrac{1}{98\cdot99\cdot100}\)

\(\Leftrightarrow2A=\dfrac{2}{1\cdot2\cdot3}+\dfrac{2}{2\cdot3\cdot4}+\dfrac{2}{3\cdot4\cdot5}+...+\dfrac{2}{98\cdot99\cdot100}\)

\(\Leftrightarrow2A=-\dfrac{1}{1\cdot2}+\dfrac{1}{2\cdot3}-\dfrac{1}{2\cdot3}+\dfrac{1}{3\cdot4}-\dfrac{1}{3\cdot4}+\dfrac{1}{4\cdot5}-\dfrac{1}{4\cdot5}+...-\dfrac{1}{98\cdot99}+\dfrac{1}{99\cdot100}\)

\(\Leftrightarrow2A=-\dfrac{1}{2}+\dfrac{1}{99\cdot100}\)

\(\Leftrightarrow2A=\dfrac{-1}{2}+\dfrac{1}{9900}\)

\(\Leftrightarrow2A=\dfrac{-4950}{9900}+\dfrac{1}{9900}=\dfrac{-4949}{9900}\)

hay \(A=\dfrac{-4949}{19800}\)

2x3x4

=6x4

=24

\(2\times3\times4=24\)

2.3.4

=24

4320 bn nhé !!!!!

tk cho mk nha bn

=4320

TK MÌNH NHÉ

 180 x (2 x 3 x 4)

=180 x 24

= 4320

tk và kb nha

Gọi A = 1.2.3 + 2.3.4 + ................... + 2011.2012.2013

4A = 1.2.3.4 + 2.3.4.4 + ...................... + 2011.2012.2013.4

4A = 1.2.3.4 + 2.3.4.(5 - 1) +..................... + 2011.2012.2013.(2014 - 2010)

4A = 1.2.3.4 + 2.3.4.5 - 1.2.3.4 + .................... + 2011.2012.2013.2014 - 2010.2011.2012.2013

4A = 2011.2012.2013.2014

A = \(\frac{2011.2012.2013.2014}{4}=2011.503.2013.2014=..........\)

4xA=1x2x3x4+2x3x4x4+3x4x5x4+...+15x16x17x4

4xA=1x2x3x4+2x3x4x(5-1)+3x4x5x(6-2)+...+-15x16x17x(18-14)

4xA=1x2x3x4-1x2x3x4+2x3x4x5-2x3x4x5+3x4x5x6-...-14x15x16x17+15x16x17x18=15x16x17x18

=> A=15x4x17x18=18360

Đặt A = 1 x 2 x 3 + 2 x 3 x 4 + 3 x 4 x 5 +....+ 98 x 99 x 100

4A = 1 x 2 x 3 x 4 + 2 x 3 x 4 x 4 + 4 x 5 x 4 +....+ 98 x 99 x 100 x 4

4A = 1 x 2 x 3 x ( 4 - 0 ) + 2 x 3 x 4 x ( 5 - 1 ) + 4 x 5 x 6 x ( 7 - 3 ) +....+ 98 x 99 x 100 x ( 101 - 97 )

4A = 1 x 2 x 3 x 4 + 2 x 3 x 4 x 5 - 1 x 2 x 3 x 4 + 4 x 5 x 6 x 7 - 3 x 4 x 5 x 6 + .... + 98 x 99 x 100 x 101 - 98 x 99 x 100 x 97

A = 98 x 99 x 100 x 97 / 4

A = 98 x 99 x 25 x 97