2.3x3,4+2,3x5,6x2,3 =2,3x3,4+2,3x5,6x2,3x1 =2,3x(3,4+5,6+1) =2,3x10 =23

Giải 

=2,3 x 3,4 + 2,3 x 5,6 + 23 x 1

= 2,3 x (3,4 + 5,6 + 1)

=2,3 x 10

=23 

23 nha bnja

`1/(2*3)+1/(3*4)+1/(4*5)+...+1/(9*10)`

`=1/2-1/3+1/3-1/4+1/4-1/5+...+1/9-1/10`

`=1/2-1/10`

`=5/10-1/10`

`=4/10=2/5`

5050 đấy bạn mình cũng không chắc lắm

 S = 1.2 + 2.3 + 3.4 + ..... + 99.100

=> 3S = 1.2.3 + 2.3(4 - 1) + 3.4(5 - 2) + ......... + 99.100(101 - 98)

=> 3S = 1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 + ........ + 99.100.101 - 98.99.100

=> 3S = (1.2.3 + 2.3.4 + 3.4.5 + ..... + 98.99.100 + 99.100.101) - (1.2.3 + 2.3.4 + .......... + 98.99.100)

=> 3S = 99.100.101

=> S = \(\frac{99.100.101}{3}=333300\)

Đặt S = 1 x 2 + 2 x 3 + 3 x 4 +... + 99 x 100

3 S = 1 x 2 x 3 + 2 x 3 x 3 + 3 x 4 x 3 + ... + 98 x 99 x 3 + 99 x 100 x 3

3 S = 1 x 2 x 3 + 2 x 3 ( 4 - 1 ) + 3 x 4 ( 5 - 2 ) + ... + 98 x 99 ( 100 - 97 ) + 99 x 100 ( 101 - 98 )

3 S = 1 x 2 x 3 + 2 x 3 x 4 - 1 x 2 x 3 + 3 x 4 x 5 - 2 x 3 x 4 + ... - 97 x 98 x 99 + 99 x 100 x 101 - 98 x 99 x 100

3 S = 99 x 100 x 101  3S = 3 x 33 x100 x 101

S = 33 x 100 x 101 = 333 300

\(\frac{1}{2\cdot3}+\frac{1}{3\cdot4}+\frac{1}{4\cdot5}+\frac{1}{5\cdot6}+\frac{1}{6\cdot7}+\frac{1}{7\cdot8}\)

\(=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}\)

\(=\frac{1}{2}-\frac{1}{8}=\frac{6}{16}=\frac{3}{8}\)

\(\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}\)

\(=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{7}-\frac{1}{8}\)

\(=\frac{1}{2}-\frac{1}{8}=\frac{3}{8}\)

\(\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}\)

\(=1\times\left(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{7}-\frac{1}{8}\right)\)

\(=1\times\left(\frac{1}{2}-\frac{1}{8}\right)\)

\(=1\times\frac{3}{8}\)

\(=\frac{3}{8}\)

2,3x4,5+2,3x2,5+7x7,7

=2,3x(4,5+2,5)+7x7,7

=2,3x7+7x7,7

=7x(2,3+7,7)

=7x10

=70

2.3 x 2.5 x 0,1 x 0.4

= ( 2.3 x 0.1 ) x ( 2.5 x 0.4 )

= 0.23 x 1

= 0.23

b) 1.23 : 1.3 x 2.6

= 1.23 x 2.6 : 1.3

= 3.198 : 1.3

= 2,46

Mk nhanh nhất đó

Đúng 100%

Tk mk mk tk lại

Cảm ơn bạn nhiều

Thank you very  much

( ^ _ ^ )

giúp mình với, mình k cho

2,3 x 2,5 x 0,1 x 0,4

= (2,3 x 0,1) x (2,5 x 0,4)

= 0,23 x 1

= 0,23

A=1.2+2.3+3.4+........+98.99 
3A=1.2.3+2.3.3+3.4.3+........+98.99.3 
3A=1.2.3+2.3.(4 -1) +3.4.(5 -2)+........+98.99.(100 -97) 
3A=1.2.3+2.3.4 -1.2.3 +3.4.5 -2.3.4 +........+98.99.100 -97.98.99 
3A=98.99.100 
===>A=(98.99.100)/3

#Japhkiel#

A=1.2+2.3+3.4+........+98.99 
3A=1.2.3+2.3.3+3.4.3+........+98.99.3 
3A=1.2.3+2.3.(4 -1) +3.4.(5 -2)+........+98.99.(100 -97) 
3A=1.2.3+2.3.4 -1.2.3 +3.4.5 -2.3.4 +........+98.99.100 -97.98.99 
3A=98.99.100 
A=\(\frac{98.99.100}{3}=\frac{970200}{3}=323400\)

Đặt A\(=1\cdot2+2\cdot3+3\cdot4+......+98\cdot99\), ta có

\(3A=1\cdot2\cdot3+2\cdot3\cdot3+.....+98\cdot99\cdot3\)

\(=1\cdot2\cdot3+2\cdot3\cdot\left(4-1\right)+....+98\cdot99\cdot\left(100-97\right)\)

\(=1\cdot2\cdot3+2\cdot3\cdot4-1\cdot2\cdot3+....+98\cdot99\cdot100-97\cdot98\cdot99\)

\(=98\cdot99\cdot100\)

\(\Rightarrow A=\frac{98\cdot99\cdot100}{3}=323400\)

=17.9 nha bn

chúc bn hok giỏi

17.9 nha ban chúa học tốt

3.4 + 3.4 + 5.6 + 7.8 - 2.3

= 12 + 12 + 30 + 56 - 6

=  17,9