a)12(x-1)=0

=>x-1=0

=>x=1

b) 3/5x-3/4=(-3)²-|-11|

3/5x-3/4=9-11=-2

3/5x=-2+3/4=-5/4

=>x=-5/4:3/5=-25/12

c)4(x-5/8)-3/4=0,25

4(x-5/8)=0,25+3/4=1

=>x-5/8=1/4

x=1/4+5/8=7/8

2) Số đó là

27:3/5=45

3: 

\(=\dfrac{1}{7}\cdot\dfrac{3}{5}\cdot\dfrac{5}{6}\cdot\dfrac{5}{8}=\dfrac{1}{7}\cdot\dfrac{1}{2}\cdot\dfrac{5}{8}=\dfrac{5}{112}\)

4:

=>2/3:x=-2-1/3=-7/3

=>x=-2/3:7/3=-2/7

5:

AC=CB=12/2=6cm

IB=6/2=3cm

a)\(x-\frac{1}{4}=\frac{5}{8}\)

\(x=\frac{5}{8}+\frac{1}{4}\)

\(x=\frac{7}{8}\)

b)\(\frac{x}{12}=\frac{-1}{24}-\frac{1}{8}\)

\(\frac{x}{12}=\frac{-1}{24}+\frac{-1}{8}\)

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

\(x=\frac{-1}{6}\times12\)

\(x=-2\)

a)x-1/4=5/8

=>x=5/8+1/4

x=7/8

b) x/12=-1/24-1/8

=>x/12=-1/6

=>x=-2

c).......... Tự làm

\(\left(0,25-30\%.x\right).\frac{1}{3}-\frac{1}{4}=-5\frac{1}{6}\)

\(< =>\left(\frac{1}{4}-\frac{3x}{10}\right).\frac{1}{3}-\frac{1}{4}=-\frac{30}{6}\)

\(< =>\left(\frac{1}{4}-\frac{3x}{10}\right).\frac{1}{3}=-5+\frac{1}{4}=-\frac{19}{4}\)

\(< =>\frac{1}{4}-\frac{3x}{10}=\frac{-19}{4}.\frac{3}{1}=-\frac{57}{4}\)

\(< =>\frac{-3x}{10}=\frac{-57}{4}-\frac{1}{4}=-\frac{58}{4}\)

\(< =>-12x=-580\)

\(< =>12x=580< =>x=\frac{580}{12}=\frac{145}{3}\)

\(\left(0,25-30\%x\right).\frac{1}{3}-\frac{1}{4}=-5\frac{1}{6}\)

\(\left(\frac{1}{4}-\frac{3}{10}x\right)\frac{1}{3}-\frac{1}{4}=\frac{-31}{6}\)

\(\frac{1}{4}-\frac{3}{10}x=\left(\frac{-31}{6}+\frac{1}{4}\right)\div\frac{1}{3}\)

\(\frac{1}{4}-\frac{3}{10}x=\frac{-59}{4}\)

\(\frac{3}{10}x=\frac{1}{4}-\frac{-59}{4}\)

\(\frac{3}{10}x=15\Leftrightarrow x=50\)

a: Ta có: \(\dfrac{1}{4}:x=3\dfrac{4}{5}:40\dfrac{8}{15}\)

\(\Leftrightarrow x=\dfrac{1}{4}\cdot\dfrac{\dfrac{608}{15}}{3+\dfrac{4}{5}}\)

\(\Leftrightarrow x=\dfrac{152}{15}:\dfrac{19}{5}=\dfrac{8}{3}\)

b: Ta có: \(\left(x+1\right):\dfrac{5}{6}=\dfrac{20}{3}\)

\(\Leftrightarrow x+1=\dfrac{50}{9}\)

hay \(x=\dfrac{41}{9}\)

c: Ta có: \(\dfrac{7}{x-1}=\dfrac{x+1}{9}\)

\(\Leftrightarrow x^2-1=63\)

\(\Leftrightarrow x^2=64\)

hay \(x\in\left\{8;-8\right\}\)

c. \(\dfrac{7}{x-1}=\dfrac{x+1}{9}\) 

    \(7.9=\left(x-1\right).\left(x+1\right)\) 

    \(63=x^2-1\) 

             \(x^2=63+1\) 

             \(x^2=64\) 

             \(x^2=8^2\)

             \(x=8\)          

Đặt \(A=\frac{1}{1\cdot3}+\frac{1}{3\cdot5}+\cdots+\frac{1}{19\cdot21}\)

\(=\frac12\left(\frac{2}{1\cdot3}+\frac{2}{3\cdot5}+\cdots+\frac{2}{19\cdot21}\right)\)

\(=\frac12\left(1-\frac13+\frac13-\frac15+\cdots+\frac{1}{19}-\frac{1}{21}\right)\)

\(=\frac12\left(1-\frac{1}{21}\right)=\frac12\cdot\frac{20}{21}=\frac{10}{21}\)

Ta có: \(\left(\frac{1}{1\cdot3}+\frac{1}{3\cdot5}+\cdots+\frac{1}{19\cdot21}\right)\cdot420-\frac{\left\lbrack0,4\left(7,5-2,5x\right)\right\rbrack}{0,25}=212\)

=>\(420\cdot\frac{10}{21}-\frac{3-x}{0,25}=212\)

=>200-4(3-x)=212

=>200+4(x-3)=212

=>4(x-3)=12

=>x-3=3

=>x=6

a) x - 1/2 = 3/5

x = 3/5 + 1/2

x = 11/10

b) x - 1/2 = -2/3

x = -2/3 + 1/2

x = -1/6

c) 2/5 - x = 0,25

x = 2/5 - 0,25

x = 2/5 - 1/4

x = 3/20

Câu 1:

a)\(\frac{3}{4}-0,25-\left[\frac{7}{3}+\left(-\frac{9}{2}\right)\right]-\frac{5}{6}\)

    \(=\frac{3}{4}-\frac{1}{4}-\frac{14}{6}+\frac{27}{6}-\frac{5}{6}\)

    \(=\frac{1}{2}-\frac{4}{3}\)

     \(=-\frac{5}{6}\)

b)\(7+\left(\frac{7}{12}-\frac{1}{2}+3\right)-\left(\frac{1}{12}+5\right)\)

    \(=7+\frac{1}{12}+3-\frac{1}{12}-5\)

    \(=5\)

Câu 2:

\(\frac{3}{4}-\frac{5}{6}\le\frac{x}{12}< 1-\left(\frac{2}{3}-\frac{1}{4}\right)\)

\(-\frac{1}{12}\le\frac{x}{12}< 1-\frac{5}{12}\)

\(-\frac{1}{12}\le\frac{x}{12}< \frac{7}{12}\)

           Vậy -1\(\le\)x<7

a) \(215+x=400\)

\(\Rightarrow x=400-215\)

\(\Rightarrow x=185\)

b) \(12,5-2x=\dfrac{1}{5}\)

\(\Rightarrow2x=12,5-\dfrac{1}{5}\)

\(\Rightarrow2x=\dfrac{123}{10}\)

\(\Rightarrow x=\dfrac{123}{10}:2\)

\(\Rightarrow x=\dfrac{123}{20}\)

còn câu c d nhanh cậu ơi