\(x+\frac{13}{12}+\frac{21}{20}+...+\frac{91}{90}\)

\(=x+1+\frac{1}{12}+1+\frac{1}{20}+...+1+\frac{1}{90}\)

\(=x+\left(1+1+1+1+1+1+1\right)+\left(\frac{1}{12}+\frac{1}{20}+...+\frac{1}{90}\right)\)

\(=x+7+\left(\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{9.10}\right)\)

\(=x+7+\left(\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{9}-\frac{1}{10}\right)\)

\(=x+7+\left(\frac{1}{3}-\frac{1}{10}\right)\)

\(=x+7+\frac{7}{30}\)

\(=x+7\frac{7}{30}\)

7va7/30

a) 3/13 - 3/2 + 10/13

= (3/13 + 10/13) - 3/2

= 1 - 3/2

= -1/2

b) 4/7 - (-2/7) - 7/3

= 4/7 + 2/7 - 7/3

= 6/7 - 7/3

= -31/21

c) 2/3 - (-1/6) + 5/4

= 2/3 + 1/6 + 5/4

= 8/12 + 2/12 + 15/12

= 25/12

a, 3/13 - 3/2 + 10/13

= 3/13 + 10/13

= 1 - 3/2 = -1/2

b,4/7 - (-2/7) - 7/3

= 4/7 + 2/7 - 7/3

= 6/7 - 7/3

= 18/21 - 14/21

= 4/21

c, 2/3 - -1/6 +5/4 

= 2/3 + 1/16 +5/4

= 128/192 + 12/192 + 240/192

= 380/192

= 95/4 

không hiểu chỗ nào hỏi tui

trình bày ra hộ tôi

7^{x + 2} + 2 . 7^{x - 1} = 345

7^{x - 1} . ( 7³ + 2 ) = 345

7^{x - 1} . 345 = 345

7^{x - 1} = 345 : 345

7^{x - 1} = 1

7^{x - 1} = 7⁰

x - 1 = 0

x = 0 + 1

x = 1

(3x-2⁴).7³=2.7⁴

=>3x-16     =2.7⁴:7³

=>3x-16     =2.7

=>3x-16      =14

=>3x           =14+16

=>3x            =30

=>x              =30:3

=>x               =10

Vậy x=10

Chúc bn học tốt

k kiểu gi

\(\left(3x-2^4\right)\cdot7^3=2\cdot7^4\)

\(\Leftrightarrow3x-32=2\cdot7^4:7^3\)

\(\Leftrightarrow3x-32=2\cdot7\)

\(\Leftrightarrow3x-32=14\)

\(\Leftrightarrow3x=46\)

\(\Leftrightarrow x=\frac{46}{3}\)

a) (-25).13.(-8).4.(-125)

= (-25).4.(-8).(-125).13

= (-100).1000.13

= 1300000

b) -(487+a-789)-(789-487-3a)

= (-487)+(-a)+789+(-789)+487+3a

= (-487)+487+789+(-789)+3a+(-a)

= 0+0+2a

= 2a

c) |2x-7|=11(Có 2 trường hợp)

TH1)2x-7=11

<=>2x=11+7

<=>2x=18

<=>x=18/2

<=>x=9

TH2)2x-7=-11

<=>2x=-11+7

<=>2x=-4

<=>x=-4/2

<=>x=-2

d)3x+12=30+x

<=>3x=x+30-12

<=>3x=x+18

<=>2x+x=x+18

<=>2x=18+x-x

<=>2x=18

<=>x=18/2

<=>x=9

k mình nhé

CHÚC BẠN HỌC GIỎI

(-25 ) . 13 . ( -8 ) . 4 . ( -125 ) 

= [ -25 . 4 ] . [ -125 . ( -8 ) ] . 13 

=     -100    .       (-1000 )         . 13 

=        100 000       . 13 

= 1 300 000

| 2x - 7 | = 11 

  2x        = 11 + 7 

  2x        = 18 

    x         =  18 : 2

    x         = 9  

a ) ( - 25 ) . 13 . ( - 8 ) . 4 . ( - 125 )

= [ ( - 25 ) . 4 ] . 13 . [ ( - 8 ) . ( - 125 ) ]

= ( - 100 ) . 13 . 1000

= - 1 300 000

Số số hạng là: (19-11)*1+1=9(số hạng) 

Có số cặp số là:9:2=4 dư 1

Vậy có 4 cặp và dư 1 số

Ta có : 11+(12+19)+(13+18)+(14+17)+(15+16)

=11+21+21+21+21

=11+(21.4)

=11+84

=95

11+12+13+14+15+16+17+18+19=(11+19)+(12+18)+(13+17)+(16+14)+15    

                                                     =30+30+30+30+15

                                                      =30*4+15                                                

                                                      =120+15                                              

                                                      =135 

11 + 12 + 13 + 14 + 15 + 16 + 17 + 18 + 19

= ( 11 + 19 ) + ( 12 + 18 ) + ( 13 + 17 ) + ( 14 + 16 ) + 15

= 30 + 30 + 30 + 30 + 15

= 135

$$[13 + 2(x-5)] \div 7 = 9$$

$$\iff 13 + 2(x-5) = 9 \times 7$$

$$\iff 13 + 2(x-5) = 63$$

$$\iff 2(x-5) = 63 - 13$$

$$\iff 2(x-5) = 50$$

$$\iff x-5 = 50 \div 2$$

$$\iff x-5 = 25$$

$$\iff x =25 + 5$$

$$\iff x = 30$$

a) \(0,5x-\frac{2}{3}x=\frac{7}{12}\)

\(\frac{1}{2}x-\frac{2}{3}x=\frac{7}{12}\)

\(\Rightarrow x\left(\frac{1}{2}-\frac{2}{3}\right)=\frac{7}{12}\)

\(x.\frac{-1}{6}=\frac{7}{12}\)

\(x=\frac{7}{12}:\frac{-1}{6}\)

\(x=\frac{-7}{2}\)

b) \(x:4\frac{1}{3}=-2,5\)

\(x:\frac{13}{3}=\frac{-5}{2}\)

\(x=\frac{-5}{2}.\frac{13}{3}\)

\(x=\frac{-65}{6}\)

c) \(5,5x=\frac{13}{15}=\frac{11}{2}x=\frac{13}{5}\)

\(x=\frac{13}{5}:\frac{11}{2}\)

\(x=\frac{26}{55}\)

d) \(\left(\frac{3x}{7}+1\right):\left(-4\right)=\frac{-1}{28}\)

\(\frac{3x}{7}+1=\frac{-1}{28}.\left(-4\right)\)

\(\frac{3x}{7}+1=\frac{1}{7}\)

\(\frac{3x}{7}=\frac{1}{7}-1\)

\(\frac{3x}{7}=\frac{-6}{7}\)

\(\frac{3x}{7}=\frac{-6}{7}\Rightarrow\frac{3.\left(-2\right)}{7}=\frac{-6}{7}\)

Vậy x = -2

\(0,5x-\frac{2}{3}x=\frac{7}{12}\)\(\)

\(\frac{1}{2}x-\frac{2}{3}x=\frac{7}{12}\)

\(x.\left(\frac{1}{2}-\frac{2}{3}\right)=\frac{7}{12}\)

\(x.\frac{-1}{6}=\frac{7}{12}\)

\(x=\frac{7}{12}:\frac{-1}{6}\)

\(x=3,5\)