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\(\frac{3}{4}x\frac{4}{5}:\frac{3}{5}x\frac{6}{7}:\frac{1}{7}x\frac{5}{6}\)\(=\)\(\frac{3}{4}x\frac{4}{5}x\frac{5}{3}x\frac{6}{7}x\frac{7}{1}x\frac{5}{6}\)\(=\)\(\frac{3x4x5x6x7x5}{4x5x3x7x1x6}\)\(=\)\(\frac{\left(3\right)x\left(4\right)x\left(5\right)x\left(6\right)x\left(7\right)x5}{\left(3\right)x\left(4\right)x\left(5\right)x\left(6\right)x\left(7\right)x1}\)\(=\)\(\frac{5}{1}\)\(=\)\(5\).
=2008x1+2008x1+2008x1+2008x1-2008x4
=2008x(1+1+1+1-4
=2008x0
=0
Đặt Tổng trên là A
A = 1/1.3 + 1/3.5 + 1/5.7 + .... + 1/2005.2007
2. A = 2 . ( 1/1.3 + 1/3.5 + 1/5.7 + .... + 1/2005.2007 )
2A = 2/1.3 + 2/3.5 + 2/5.7 + ..... + 2/2005.2007
2A = 1 - 1/3 + 1/3 - 1/5 + 1/5 - 1/7 + ..... + 1/2005 - 1/2007
2A = 1 - 1/2007
2A = 2006/2007
A = 2006/2007 : 2
A = 2006/4014
- Hok Tot -
\(\dfrac{1}{1\times3}+\dfrac{1}{3\times5}+\dfrac{1}{5\times7}+....+\dfrac{1}{2005\times2007}\)
= \(\dfrac{1}{2}\left(\dfrac{1}{1}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+...+\dfrac{1}{2005}-\dfrac{1}{2007}\right)\)
= \(\dfrac{1}{2}\times\left(\dfrac{1}{1}-\dfrac{1}{2007}\right)\)
= \(\dfrac{1}{2}\times\dfrac{2006}{2007}\)
= \(\dfrac{1003}{2007}\)
25x9x4x100:1000
=(25x4)x9x100:1000
=100x9x100:1000
=10000:1000x9
=10x9
=90
20x190x50
=(20x50)x190
=100x190
=19000
(900x28):9
=(900:9)x28
=100x28
=2800
20 x 120 x 50
= [ 20 x 50 ] x 120
= 100 x 50
= 19000
[ 900 x 28 ] : 9
= [ 900 : 9 ] : 29
= 100 x 28
= 2800
=
(24x5)x(3x10)
=120x30
=3600
= 3600