Lời giải:
$2,5\times 20,23\times 8\times 5=2,5\times 20,23\times 4\times 2\times 5$

$=(2,5\times 4)\times 20,23\times (2\times 5)$

$=10\times 20,23\times 10=20,23\times 100=2023$

    20,23 \(\times\) 37 - 2,3 \(\times\) 5,5 + 20,23 \(\times\) 63 - 2,3 \(\times\) 4,5

= ( 20,23 \(\times\) 37 + 20,23 \(\times\) 63 ) - ( 2,3 \(\times\) 5,5 + 2,3 \(\times\) 4,5)

= 20,23 \(\times\) ( 37 + 63) - 2,3 \(\times\) ( 5,5 + 4,5)

= 20,23 \(\times\) 100 - 2,3 \(\times\) 10

= 2023 - 23

= 2000

a: \(20,23\cdot7,5+20,23+20,23+20,23:2\)

\(=20,23\cdot7,5+20,23\cdot2+20,23\cdot0,5\)

\(=20,23\left(7,5+2+0,5\right)\)

\(=20,23\cdot10=202,3\)

b: \(100:9-79,3:9\)

\(=\left(100-79,3\right):9\)

\(=20,7:9=2,3\)

a, = \(\frac{11}{21}\)x \(\left(\frac{7}{4}+\frac{5}{4}\right)\)

   = \(\frac{11}{21}\)x 3

  = \(\frac{33}{21}\)

b) = \(\left(\frac{23}{14}-\frac{9}{14}\right)\)x \(\frac{6}{13}\)

    = 1 x \(\frac{6}{13}\)

    = \(\frac{6}{13}\)

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20,23 : 0,25 + 20,23 : 0,1 + 20,23 + 85 x 20,23

= 20,23 x 4 + 20,23 x 10 + 20,23 x 1 + 85 x 20,23

= 20,23 x (4 + 10 + 1 + 85)

= 20,23 x 100

= 2023

 20,23 : 0,25 + 20,23 : 0,1 + 20,23 + 85 x 20,23

= 20,23 x 4 + 20,23 x 10 + 20,23 + 85 x 20,23

= 20,23 x ( 4 + 10 + 1 + 85 )

= 20,23 x 100

= 2023

13/19 x 23/11 - 13/19 x 8/11 - 13/19 x 4/11

= 13/19 x (23/11 - 8/11 - 4/11)

= 13/19 x 1

= 13/19

\(\dfrac{13}{19}\times\dfrac{23}{11}-\dfrac{13}{19}\times\dfrac{8}{11}-\dfrac{13}{19}\times\dfrac{4}{11}\\ =\dfrac{13}{19}\times\left(\dfrac{23}{11}-\dfrac{8}{11}-\dfrac{4}{11}\right)\\ =\dfrac{13}{19}\times1=\dfrac{13}{19}\)

câu b, 20/5x7 + 20/7x9 + 20/ 9 x11 + ..... 20 / 33 x35

21/11 x 22/17x68/63

= 7x3x2x11x4x17      /           11x17x7x3x3

=8/3

5/14x7/13x26/25

=5x7x26              /                14x13x25

= 5x7x13x2             /              7x2x13x5x5

=1/5

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