Bài 2:

a: Ta có: \(2x+79=x+45\)

nên 2x-x=45-79

hay x=-24

b: Ta có: \(15-\left(x+7\right)=\left(x-8\right)+22\)

\(\Leftrightarrow8-x-x-14=0\)

\(\Leftrightarrow2x=22\)

hay x=11

a.\(x+\dfrac{4}{7}=\dfrac{38}{21}\)

\(x=\dfrac{38}{21}-\dfrac{4}{7}\)

\(x=\dfrac{38}{21}-\dfrac{12}{21}=\dfrac{26}{21}\)

b.\(x-\dfrac{1}{3}=\dfrac{7}{45}:\dfrac{2}{15}\)

\(x-\dfrac{1}{3}=\dfrac{7}{6}\)

\(x=\dfrac{7}{6}+\dfrac{1}{3}\)

\(x=\dfrac{7}{6}+\dfrac{2}{6}=\dfrac{9}{6}\)

x = 38/21 - 4/7

x = 26/21

x - 1/3 = 7/6

x = 7/6 + 1/3

x = 3/2

\(a,50\%x-0,2+x=\dfrac{4}{5}\)

\(\Leftrightarrow\dfrac{1}{2}x-0,2+x=\dfrac{4}{5}\)

\(\Leftrightarrow\dfrac{1}{2}x+x=\dfrac{4}{5}+0,2\)

\(\Leftrightarrow\dfrac{3}{2}x=\dfrac{4}{5}+\dfrac{1}{5}\)

\(\Leftrightarrow\dfrac{3}{2}x=1\)

\(\Leftrightarrow x=\dfrac{2}{3}\)

\(b,\left(x-\dfrac{3}{4}\right):\dfrac{1}{2}+\dfrac{3}{2}=\dfrac{25}{2}\)

\(\Leftrightarrow\left(x-\dfrac{3}{4}\right).2=\dfrac{25}{2}-\dfrac{3}{2}\)

\(\Leftrightarrow\left(x-\dfrac{3}{4}\right).2=\dfrac{22}{2}\)

\(\Leftrightarrow x-\dfrac{3}{4}=11:2\)

\(\Leftrightarrow x=\dfrac{11}{2}+\dfrac{3}{4}\)

\(\Leftrightarrow x=\dfrac{25}{4}\)

a; Áp dụng tính chất của dãy tỉ số bằng nhau, ta được:

\(\dfrac{x}{7}=\dfrac{y}{4}=\dfrac{x-y}{7-4}=\dfrac{12}{3}=4\)

Do đó: x=28; y=16

\(a,\dfrac{x}{7}=\dfrac{y}{4}=\dfrac{x-y}{7-4}=\dfrac{12}{3}=4\\ \Rightarrow\left\{{}\begin{matrix}x=4.7=28\\y=4.4=16\end{matrix}\right.\\ b,\dfrac{x}{2}=\dfrac{y}{3};\dfrac{y}{2}=\dfrac{z}{2}\Rightarrow\dfrac{x}{4}=\dfrac{y}{6}=\dfrac{z}{6}=\dfrac{x+y+z}{4+6+6}=\dfrac{50}{16}=\dfrac{25}{8}\\ \Rightarrow\left\{{}\begin{matrix}x=\dfrac{25}{8}.4=\dfrac{25}{2}\\y=\dfrac{25}{8}.6=\dfrac{75}{4}\\z=\dfrac{25}{8}.6=\dfrac{75}{4}\end{matrix}\right.\)

a: x=2/3-4/5=10/15-12/15=-2/15

b: 1/2-x=7/12

=>x=1/2-7/12=-1/12

c: =>7/2:x=-7/2

=>x=-1

d: =>1/6x=3/8-5/2=3/8-20/8=-17/8

=>x=-17/8*6=-102/8=-51/4

e: =>1,5x=-1,5

=>x=-1

\(a,\Leftrightarrow x\left(12+7+1\right)=100\)

\(\Leftrightarrow x.20=100\)

\(\Leftrightarrow x=50\)

\(b,\Leftrightarrow x\left(4+3\right)=1071\)

\(\Leftrightarrow7x=1071\)

\(\Leftrightarrow x=153\)

\(c,\Leftrightarrow x-60720=960\times5\)

\(\Leftrightarrow x-60720=4800\)

\(\Leftrightarrow x=4800+60720\)

\(\Leftrightarrow x=65520\)

\(d,\Leftrightarrow x:4=22850-11250\)

\(\Leftrightarrow x:4=11600\)

\(\Leftrightarrow x=46400\)

gõ latex đi bn!

a. 2^x + 70 = 74

<=> 2^x = 4

<=> x = 2

b. 120 - \(\dfrac{4x}{2}\) = 80

<=> 120 - 2x = 80

<=> 120 - 80 = 2x

<=> 2x = 40

<=> x = 20

c. (3x + 5)² = 400

<=> \(|3x+5|=\sqrt{400}\)

<=> \(|3x+5|=20\)

<=> \(\left[{}\begin{matrix}3x+5=20\\3x+5=-20\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=5\\x=\dfrac{-25}{3}\end{matrix}\right.\) 

a: Ta có: \(71-\left(x+33\right)=26\)

\(\Leftrightarrow x+33=45\)

hay x=12

b: Ta có: \(\left(x+73\right)-26=76\)

\(\Leftrightarrow x+73=102\)

hay x=29

c: Ta có: \(45-\left(x+9\right)=6\)

\(\Leftrightarrow x+9=39\)

hay x=30

a) 71 - ( 33 + x ) = 26

(33 + x ) = 71 -26

33 + x = 45

x        = 45 - 33 = 12

b) ( x + 73 ) - 26 = 76

    ( x + 73 )        = 102

    x                     = 102 - 73 = 29

c) 45 - ( x + 9 ) = 6

 ( x + 9 )           = 45 - 6 = 39

 x                      = 39 - 9 = 30

d) 89 - ( 73 - x ) = 20 

 73 - x  = 89 - 20

73 - x = 69

x          = 73 - 69 = 4

e) 4(x+41) = 400

   x + 41 = 400 : 4 = 100

   x = 100 - 41 = 59

f) 11(x-9 ) = 77

     x - 9 = 77 : 11 = 7

    x = 7 + 9 = 16

g) x + 7 = 25 + 13 = 38

    x = 38 - 7 = 31

h) x + 4 = 198 - 120 = 78

   x = 78 - 4 = 74

i) x - 9 = 350 : 5 = 70

   x = 70 + 9 = 79

j) 2x - 49 = 5 . 9 = 45

   2x = 45 + 49 = 94

   x = 94 : 2 = 47

k) 25 + 3( x - 8 ) = 106

    3(x-8 ) = 106 - 25 = 81

   x - 8 = 81 : 3 = 27 

x = 27 +8= 35

l) 9( x + 4 ) - 25 = 20

   9( x + 4 ) = 20 + 25 = 45

     x + 4 =45 : 9 = 5

   x = 5 - 4 = 1

m) 200 - ( 2x + 6 ) = 64

     2x + 6 = 200 - 64 = 136

   2x = 136 - 6 = 130

x = 130 : 2 = 65

n) 2(x-51) = 16 + 20 = 36

   x - 51 = 36 : 2 = 13

  x = 13 + 51 = 64

o) 450 : ( x - 19 ) = 50

  x - 19 = 450 : 50 = 9

x = 19 + 9 = 28

p)4(x-3) = 49 - 1 = 48

x - 3 = 48 : 4 = 12

x + 15

q) 156 - ( x+61 ) = 82

x + 61 = 156 - 82 = 74

x = 74 - 61 = 13

r) ( x - 35 ) - 120 = 0

x - 35 = 120

x = 120 + 35 = 155

Bài 2: Tính bằng cách thuận tiện nhất:
a) 239 x 45 + 55 x 239                            b) 459 x 107 – 7 x 459
=  239 x (45 + 55)                                    =  459 x (107 - 7)
=  239 x     100                                         =  495 x    100
=      23900                                               =      49500

c) 784 x 98 +784 +784                            d) 982 x 1003 – 982 x2 – 982
=  784 x 98 + 784 x 2                              =  982 x (1003 -2 -1)
=  784 x (98+2)                                        =  982 x    1000
=  784 x   100                                          =   982000
=      78400