a) 35 x 2 x 9 x 5

= (35 x 9) x (2 x 5)

= (35 x 10 - 35) x 10

= (350 - 35) x 10

= 315 x 10 = 3150

b) 24 x 71 + 71 x 75 + 71

= 71 x (24+75+1)

=71 x 100

= 7100

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a) \(\dfrac{3}{5}\times\dfrac{17}{21}+\dfrac{17}{21}\times\dfrac{2}{5}\)

\(=\dfrac{17}{21}\times\left(\dfrac{3}{5}+\dfrac{2}{5}\right)\)

\(=\dfrac{17}{21}\times1\)

\(=\dfrac{17}{21}\)

b) \(\dfrac{15}{8}:\dfrac{5}{20}-\dfrac{7}{8}:\dfrac{1}{4}\)

\(=\dfrac{15}{8}\times\dfrac{20}{5}-\dfrac{7}{8}\times4\)

\(=\dfrac{15}{8}\times4-\dfrac{7}{8}\times4\)

\(=4\times\left(\dfrac{15}{8}-\dfrac{7}{8}\right)\)

\(=4\times1\)

\(=4\)

a, =(3/5+2/5)x17/21

=1x17/21

=17/21

a) \(\dfrac{3}{5}\) x \(\dfrac{17}{21}+\dfrac{17}{21}\) x \(\dfrac{2}{5}\)

\(=\left(\dfrac{3}{5}+\dfrac{2}{5}\right)\) x \(\dfrac{17}{21}\)

\(=\dfrac{5}{5}\) x \(\dfrac{17}{21}\)

\(=1\) x \(\dfrac{17}{21}\)

\(=\dfrac{17}{21}\)

b) \(\dfrac{15}{8}:\dfrac{5}{20}-\dfrac{7}{8}:\dfrac{1}{4}\)

\(=\dfrac{15}{8}:\dfrac{1}{4}-\dfrac{7}{8}:\dfrac{1}{4}\)

\(=\left(\dfrac{15}{8}-\dfrac{7}{8}\right):\dfrac{1}{4}\)

\(=\dfrac{8}{8}:\dfrac{1}{4}\)

\(=1:\dfrac{1}{4}\)

\(=\dfrac{1}{1}:\dfrac{1}{4}\)

\(=\dfrac{1}{1}\) x \(\dfrac{4}{1}\)

\(=\dfrac{4}{1}\)

\(=4\)

a, 2 x 9 x 5 x 7 x 4 x 25

= 2³ x 3² x 5³ x 7 

= 63000

b, 6 x 14 x 5 x 25 x 3 x 4

= 2⁴ x 3² x 5³ x 7

= 126000

a) 2x9x5x7x4x25
= (2x5)x(4x25)x(9x7)
= 10x100x63
= 63000
b) 2x14x5x25x3x4
= (2x5)x(4x25)x(14x3)
= 10x100x42
= 42000

Bài 2: Tính bằng cách thuận tiện nhất

\(\text{a. 1255 + 436 + 145}\)

\(=\left(1255+145\right)+436\)

\(=1400+436\)

\(=1836\)

\(\text{b. 142 x 12 + 142 x 18}\)

\(=142\times\left(12+18\right)\)

\(=142\times30\)

\(=4260\)

\(\text{c. 4 x 18 x 25}\)

\(=\left(25\times4\right)\times18\)

\(=100\times18\)

\(=1800\)

Bài 2:

a: \(=1400+436=1836\)

b: =142(12+18)

=142*30

=4260

c: \(=100\cdot18=1800\)

Bài 3:

a: \(41m^219dm^2>409dm^2\)

b: \(90000cm^2< 90m^2\)

c: \(30m^26dm^2< 3060dm^2\)

d: \(8012cm^2>8dm^212cm^2\)

e: \(5dm^29cm^2=509cm^2\)

f: \(3940dm^2=39m^240dm^2\)

g: \(300m^26dm^2>3006dm^2\)

h: \(2009cm^2< 21dm^2\)

`4,86xx0,25xx40`

`=4,86xx10`

`=48,6`

__

`79,9xx99+72+0,9` Sửa `72,99xx99+72+0,9`

`72,9 xx 99 + 72 + 0,9`

`= 72,9 xx 0,9 + (72 + 0,9)`

`= 72,9 xx 0,9 + 72,9`

`= 72,9 xx (99+1)`

`= 72,9 xx 100`

`= 7290`

\(4,86\times0,25\times40=4,86\times\left(0,25\times40\right)=4,86\times10=48,6\)
\(79,9\times99+79+0,9\)
\(=79,9\times99+79,9\times1=79,9\times\left(99+1\right)=79,9\times100=7990\)
Phần b sửa đề thành 79 thì mới hợp lý nha bn.

a) 4,86 x 0,25 x 40

= 4,86 x 10

= 48,6

b) 72,9 x 99 + 72 + 0,9

= 72,9 x 99 + 72,9

= 72,9 x 99 + 72,9 x 1

= 72,9 x (99 + 1)

= 729

a) = (360 + 126 + 123) : 3 = 203

b) = (436 + 164 - 200) : 4 = 100

c) = (525 + 120 - 145) : 5 = 100

\(a,1999-2000+1=\left(1999+1\right)-2000=2000-2000=0\\ b,450:5+550:5=\left(450+550\right):5=1000:5=200\\ c,188\times7+188\times4-188=188\times\left(7+4-1\right)=188\times10=1880\)

a,1999−2000+1=(1999+1)−2000=2000−2000=0b,450:5+550:5=(450+550):5=1000:5=200c,188×7+188×4−188=188×(7+4−1)=188×10=1880

a) 1999 - 2000 + 1

= ( 1999 + 1 ) - 2000

= 2000 - 2000

= 0

b) 450 : 5 + 550 : 5

= ( 450 + 550 ) : 5 

= 1000 : 5

= 200

c) 188 x 7 + 188 x 4 - 188

= ( 7 + 4 - 1 ) x 188

= 10 x 188

= 1880

\(\dfrac{8}{13}+\dfrac{4}{9}+\dfrac{1}{3}+\dfrac{5}{13}+3\)

\(=1+\dfrac{7}{9}+3\)

\(=4+\dfrac{7}{9}=\dfrac{43}{9}\)

`39/16 : 5/8 - 7/16 : 5/8`

`=39/16 xx 8/5- 7/16 xx 8/5`

`=8/5 xx (39/16 - 7/16)`

`= 8/5 xx2`

`= 16/5`

__

` 4/3 xx 9/8 - 4/3 xx 3/8`

`=4/3 -(9/8 - 3/8)`

`= 4/3 xx 3/4`

`=1`

\(a,\\ \dfrac{39}{16}:\dfrac{5}{8}-\dfrac{7}{16}:\dfrac{5}{8}\\ =\dfrac{39}{16}\cdot\dfrac{8}{5}-\dfrac{7}{16}\cdot\dfrac{8}{5}\\ =\dfrac{8}{5}\cdot\left(\dfrac{39}{16}-\dfrac{7}{16}\right)=\dfrac{8}{5}\cdot2=\dfrac{16}{5}\\ b,\\ \dfrac{4}{3}\cdot\dfrac{9}{8}-\dfrac{4}{3}\cdot\dfrac{3}{8}=\dfrac{3}{2}-\dfrac{1}{2}=1\)

a)

`39/16:5/8-7/16:5/8`

`=(39/16-7/16):5/8`

`=32/16xx8/5`

`=2xx8/5`

`=16/5`

b)

`4/3xx9/8-4/3xx3/8`

`=4/3xx(9/8-3/8)`

`=4/3xx6/8`

`=4/3xx3/4`

`=1`