x : 1/3 = 1/2 +1/3

=>x:1/3=5/6

=>x=5/6 x 1/3

=>x=5/18

(x:2/3):2/5=7/10

=>x:2/3=7/10 x 2/5 

=>x:2/3=7/25

=>x=7/25x2/3

=>x=14/75

CHÚC BẠN HỌC TỐT

cảm ơn bn nha!

cái này vừa gửi rồi mà:D

cách làm đấy cô ko cho >:(

1) \(x:\dfrac{1}{3}=\dfrac{1}{2}+\dfrac{1}{3}\)

\(\Rightarrow3\times x=\dfrac{5}{6}\Rightarrow x=\dfrac{5}{18}\)

2) \(\left(x:\dfrac{2}{3}\right):\dfrac{2}{5}=\dfrac{7}{10}\)

\(\Rightarrow\dfrac{3}{2}\times x=\dfrac{7}{10}\times\dfrac{2}{5}=\dfrac{7}{25}\)

\(\Rightarrow x=\dfrac{7}{25}:\dfrac{3}{2}=\dfrac{14}{75}\)

em cảm ơn 

\(x-\dfrac{1}{4}=\dfrac{-21}{10}\)

\(x=\dfrac{-21}{10}+\dfrac{1}{4}\)

\(x=\dfrac{-42}{20}+\dfrac{5}{20}=\dfrac{-37}{20}\)

Bài 2: 

Ta có: \(x\left(x-4\right)-x^2+8=0\)

\(\Leftrightarrow x^2-4x-x^2+8=0\)

\(\Leftrightarrow-4x=-8\)

hay x=2

1)=x²-49-x²

=-49

2)=>x²-4x-x²+8=0

=>-4x+8=0

=>-4x=-8

=>x=2

\(6x\left(1-3x\right)+9x\left(2x-7\right)+171=0\)

\(\Leftrightarrow6x-18x^2+18x^2-63x+171=0\)

\(\Leftrightarrow-57x=-171\)

\(\Leftrightarrow x=3\)

\(\frac{x+1}{2015}+\frac{x+2}{2014}=\frac{x+3}{2013}+\frac{x+4}{2012}\)

\(\Leftrightarrow\left(\frac{x+1}{2015}+1\right)+\left(\frac{x+2}{2014}+1\right)-\left(\frac{x+3}{2013}+1\right)-\left(\frac{x+4}{2012}+1\right)=0\)

\(\Leftrightarrow\)\(\frac{x+2016}{2015}+\frac{x+2016}{2014}-\frac{x+2016}{2013}+\frac{x+2016}{2012}=0\)

\(\Leftrightarrow\left(x+2016\right)\left(\frac{1}{2015}+\frac{1}{2014}-\frac{1}{2013}-\frac{1}{2012}\right)=0\)

\(\Leftrightarrow x+2016=0\) ( vì \(\frac{1}{2015}+\frac{1}{2014}-\frac{1}{2013}-\frac{1}{2012}\ne0\) )

\(\Leftrightarrow x=-2016\)

x = 22 x 7/11

x = 14

3/2 x X = 11/2 - 7/4

3/2 x X = 22/7

X = 22/7 : 3/2

X = 44/21

Nhanh qá='))

1. \(A=\left(2^{2017}\cdot3+2^{2017}\cdot5\right):2^{2018}\)

\(A=\left[2^{2017}.\left(3+5\right)\right]:\left(2^{2018}\right)\)

\(A=\left[2^{2017}.2^3\right]:\left(2^{2018}\right)\)

\(A=2^{2020}:2^{2018}=2^2=4\)

2. a) 2 + x : 5 = 6

=> x : 5 = 4

=> x = 20

b) 5x(7 + 48:x) = 45

=> x(7 + 48:x) = 9

=> 7x + 48 = 9

=> 7x = -39

=> x = -39/7.

c) Không hiểu đề câu này cho lắm.

3. \(25^{30}=\left(5^2\right)^{30}=5^{60};125^{19}=\left(5^3\right)^{19}=5^{57}\)

Vì 60 > 57 => \(25^{30}>125^{19}\)

4. \(S=1+7^1+...+7^{100}\)

\(\Rightarrow7S=7+7^2+...+7^{101}\)

\(\Rightarrow7S-S=7+7^2+...+7^{101}-1-7-...-7^{100}\)

\(\Rightarrow6S=7^{101}-1\)

\(\Rightarrow S=\frac{7^{101}-1}{6}\)

5. \(Q=1+2+2^2+...+2^{49}\)

\(\Rightarrow2Q=2+2^2+...+2^{50}\)

\(\Rightarrow2Q-Q=2+2^2+...+2^{50}-1-2-...-2^{49}\)

\(\Rightarrow Q=2^{50}-1\)

\(\Rightarrow2^{50}-1+1=2^n\)

\(\Rightarrow2^{50}=2^n\Rightarrow n=50\)

Tìm x :

2,8 : x - 1,5 = 2,3 : 5,75

=> x - 1,5 = 5,75

     x         = 5,75 + 1,5

     x         = 7,25

Tính nhanh :

\(\frac{1}{1\times2}+\frac{1}{2\times3}+\frac{1}{3\times4}+...+\frac{1}{13\times14}\)

\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{13}-\frac{1}{14}\)

\(=1-\frac{1}{14}+0+...+0\)

\(=\frac{13}{14}\)

1. 2,8 : x - 1,5 = 2,3 : 5,75

    2,8 : x - 1,5 = 0,4

    2,8 : x = 0,4 + 1,5

    2,8 : x = 1,9

            x = 2,8 : 1,9

            x = 1,473

2. \(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{13.14}\)

= \(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{13}-\frac{1}{14}\)

= \(1-\frac{1}{14}\)

= \(\frac{13}{14}\)