a/\(240\times20-\left(846+202\times2,5\right)\)
\(=4800-\left(846+505\right)\)
\(=4800-1351\)
\(=3449\)
b/\(2000+\left(32,4:3-2,8\right)\)
\(=2000+\left(10,8-2,8\right)\)
\(=2000+8\)
\(=2008\)
c/\(320\times1,25-\left(933+302\times3,5\right)\)
\(=400-\left(933+1057\right)\)
\(=400-1990\)
\(=-1590\)

=23+698+23+302=1046

(23+698)-(-23-302)=23+689-23+302

                               =(-23+23)+(689+302)

                               =1000

(23 + 698) - (- 23- 302)

= 23 + 698 - 23 + 302

= 698 + 302 = 1000.

\(\frac{1}{1\cdot3}+\frac{1}{3\cdot5}+\frac{1}{5\cdot7}+....+\frac{1}{99\cdot101}\)

\(=2\cdot\frac{1}{2}\cdot\left(\frac{1}{1\cdot3}+\frac{1}{3\cdot5}+\frac{1}{5\cdot7}+...+\frac{1}{99\cdot101}\right)\)

\(=\frac{1}{2}\cdot\left(\frac{2}{1\cdot3}+\frac{2}{3\cdot5}+\frac{2}{5\cdot7}+...+\frac{2}{99\cdot101}\right)\)

\(=\frac{1}{2}\cdot\left(1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{99}-\frac{1}{101}\right)\)

\(=\frac{1}{2} \cdot\left(1-\frac{1}{101}\right)\)

\(=\frac{1}{2}\cdot\frac{100}{101}\)

\(=\frac{50}{101}\)

\(B=\frac{2^2}{1.3}+\frac{2^2}{3.5}+...+\frac{2^2}{195.197}=2\left(\frac{2}{1.3}+\frac{2}{3.5}+...+\frac{2}{195.197}\right)\)
\(=2\left(1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+...+\frac{1}{195}-\frac{1}{197}\right)=2\left(1-\frac{1}{197}\right)=2.\frac{196}{197}=\frac{392}{197}\)

4/3.5+4/5.7+4/7.9+4/9.11

=4.(1/3.5+1/5.7+1/7.9+1/9.11)

=4.1/2.(2/3.5+2/5.7+2/7.9+2/9.11)

=2.(1/3-1/5+1/5-1/7+1/7-1/9+1/9-1/11)

=2.(1/3-1/11)

=2.8/33

=16/33

  4/3.5+4/5.7+4/7.9+4/9.11

=4.2/2.3.5+4.2/2.5.7+4.2/2.7.9+4.2/2.9.11

=4/2.2/3.5+4/2.2/5.7+4/2.2/7.9+4/2.2/9.11

=4/2.(2/3.5+2/5.7+2/7.9+2/9.11)

=4/2.(1/3-1/5+1/5-1/7+1/7-1/9+1/9-1/11)

=2.(1/3-1/11)

=2.8/33

=16/33

16/33 nha

a :

[( 10 - x ) . 2 + 5] : 3 - 2 = 3

( 10 - x ) x 2 + 5 = ( 3+2) x 3

( 10 - x ) x 2 + 5 = 15

10 -x  = ( 15 -5 ) : 2

10 - x = 5

x = 5

a) \(\left[\left(10-x\right)2+5\right]:3-2=3\)

\(\left(20-2x+5\right):3-2=3\)

\(\left(20-2x+5\right):3=5\)

\(20-2x+5=15\)

\(20-2x=10\)

\(2x=10\)

\(x=5\)

b) \(6x-302=2^3\cdot5\)

\(6x=8\cdot5+302\)

\(6x=342\)

\(x=57\)

c) \(12\left(x-1\right):3=4^3-2^3\)

\(12\left(x-1\right):3=56\)

\(12\left(x-1\right)=168\)

\(x-1=12\)

\(x=13\)

a, [(10-x).2+5]:3-2=3

    [(10-x).2+5]:3  =3+2

    [(10-x).2+5]:3  =5

    (10-x).2+5       =5.3

    (10-x).2+5       =15

    (10-x).2           =15-5

    (10-x).2           =10

    10-x                =10:2

    10-x                =5

        x                =10-5

        x                =5

b, 6x-320=2³.5

    6x-320=8.5

    6x-320=40

    6x      =40+320

    6x      =360

     x       =360:6

     x       =60

c, 12.(x-1):3=4³-2³

    12.(x-1):3=64-8

    12.(x-1):3=56

    12.(x-1)   =56.3

    12.(x-1)   =168

          x-1   =168:12

          x-1   =14

          x      =14+1

          x      =15

thêm gì vào số thứ 1 vậy bạn

thêm số 5 vào bên phải ấy mà

Khi thêm số 5 vào số thứ 1 ta được số thứ 2. vì thế số thứ 1 gấp 10 lần số thứ 2 :

ST1 = a                         vậy ta có sơ đồ       ST1 I---I

ST2 = a5                                                      ST2  I---I---I---I---I---I---I---I---I---I---I

ST1 = ( 302 - 5 ) : (10 + 1) 

                297     :      11          = 27                                        vậy a = 27

ST2= a5=275

  ST1=27

  ST2=275