`@` `\text {Ans}`

`\downarrow`

`a)`

`210 \div x - 1/2 = 20,5`

`=> 210 \div x = 20,5 + 1/2`

`=> 210 \div x =21`

`=> x = 210 \div 21`

`=> x = 10`

Vậy, `x = 10.`

`b)`

`7 * 3^x + 20*3^x = 3^25`

`=> 3^x * (7+20) = 3^25`

`=> 3^x * 27 = 3^25`

`=> 3^x * 3^3 = 3^25`

`=> 3^x = 3^25 \div 3^3`

`=> 3^x = 3^22`

`=> x = 22`

Vậy, `x = 22.`

a) \(210:x-\dfrac{1}{2}=20,5\)

\(\Rightarrow210:x=20,5+\dfrac{1}{2}\)

\(\Rightarrow210:x=21\)

\(\Rightarrow x=\dfrac{210}{21}\)

\(\Rightarrow x=10\)

b) \(7\cdot3^x+20\cdot3^x=3^{25}\)

\(\Rightarrow3^x\cdot\left(7+20\right)=3^{25}\)

\(\Rightarrow3^x\cdot27=3^{25}\)

\(\Rightarrow3^x\cdot3^3=3^{25}\)

\(\Rightarrow3^{x+3}=3^{25}\)

\(\Rightarrow x+3=25\)

\(\Rightarrow x=25-3\)

\(\Rightarrow x=22\)

Bài 4: 

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

\(1,25-x=2,25-\dfrac{4}{3}=\dfrac{9}{4}-\dfrac{4}{3}\)

\(1,25-x=\dfrac{11}{12}\)

\(x=1,25-\dfrac{11}{12}=\dfrac{5}{4}-\dfrac{11}{12}\)

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

b) \(\dfrac{17}{6}-\left(x-\dfrac{7}{6}\right)=\dfrac{7}{4}\)

\(x-\dfrac{7}{6}=\dfrac{17}{6}-\dfrac{7}{4}=\dfrac{34}{12}-\dfrac{21}{12}\)

\(x-\dfrac{7}{6}=\dfrac{13}{12}\)

\(x=\dfrac{13}{12}+\dfrac{7}{6}=\dfrac{13}{12}+\dfrac{14}{12}\)

\(x=\dfrac{27}{12}=\dfrac{9}{4}\)

c) \(4-\left(2x+1\right)=3-\dfrac{1}{3}=\dfrac{9}{3}-\dfrac{1}{3}\)

\(4-\left(2x+1\right)=\dfrac{8}{3}\)

\(2x+1=\dfrac{8}{3}+4=\dfrac{8}{3}+\dfrac{12}{3}\)

\(2x+1=\dfrac{20}{3}\)

\(2x=\dfrac{20}{3}-1=\dfrac{20}{3}-\dfrac{3}{3}\)

\(2x=\dfrac{17}{3}\)

\(x=\dfrac{17}{3}.\dfrac{1}{2}=\dfrac{17}{6}\)

Bài 15:

a) \(\left(\dfrac{-2}{3}\right)^9:x=\dfrac{-2}{3}\)

\(x=\left(\dfrac{-2}{3}\right)^9:\dfrac{-2}{3}=\left(\dfrac{-2}{3}\right)^{9-1}\)

\(=>x=\left(\dfrac{-2}{3}\right)^8\)

b) \(x:\left(\dfrac{4}{9}\right)^5=\left(\dfrac{4}{9}\right)^4\)

\(x=\left(\dfrac{4}{9}\right)^4.\left(\dfrac{4}{9}\right)^5=\left(\dfrac{4}{9}\right)^{4+5}\)

\(=>x=\left(\dfrac{4}{9}\right)^9\)

c) \(\left(x+4\right)^3=-125\)

\(\left(x+4\right)^3=\left(-5\right)^3\)

\(=>x+4=-5\)

\(x=-5-4\)

\(=>x=-9\)

d) \(\left(10-5x\right)^3=64\)

\(\left(10-5x\right)^3=4^3\)

\(=>10-5x=4\)

\(5x=10-4\)

\(5x=6\)

\(=>x=\dfrac{6}{5}\)

e) \(\left(4x+5\right)^2=81\)

\(\Leftrightarrow\left[{}\begin{matrix}\left(4x+5\right)^2=\left(-9\right)^2\\\left(4x+5\right)^2=9^2\end{matrix}\right.\)

\(\Leftrightarrow\left[{}\begin{matrix}4x+5=-9\\4x+5=9\end{matrix}\right.\)

\(\Leftrightarrow\left[{}\begin{matrix}4x=-14\\4x=4\end{matrix}\right.\)

\(\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{-14}{4}\\x=1\end{matrix}\right.\)

Bài 16:

a) \(4-1\dfrac{2}{5}-\dfrac{8}{3}\)

\(=4-\dfrac{7}{5}-\dfrac{8}{3}\)

\(=\dfrac{60-21-40}{15}=\dfrac{-1}{15}\)

b) \(-0,6-\dfrac{-4}{9}-\dfrac{16}{15}\)

\(=\dfrac{-3}{5}+\dfrac{4}{9}-\dfrac{16}{15}\)

\(=\dfrac{\left(-27\right)+20-48}{45}=\dfrac{-55}{45}=\dfrac{-11}{9}\)

c) \(-\dfrac{15}{4}.\left(\dfrac{-7}{15}\right).\left(-2\dfrac{2}{5}\right)\)

\(=\dfrac{7}{4}.\dfrac{-12}{5}\)

\(=\dfrac{-21}{5}\)

\(#Wendy.Dang\)

Uh, chừa sau k dám học muộn nx

Bạn ơi, bạn viết lại đề đi. Khó nhìn quá

ok bạn 

a)-7.(x+9)-3.(5-x)=2

-7.x-63-15+3.x=2

-4.x-78=2

-4.x=2+78=80

x=80:(-4)

x=-20

b)3.(x+2)-6.(x-5)=2.(5-2x)

3x+6-6x+30=10-4.x²

9x+36=6.x²

Tới đây bn tự làm nha vì mk bí ý tưởng rùi

\(-\frac{4}{9}+\left(-\frac{5}{6}\right)-\frac{17}{4}=-\frac{199}{36}\)

\(-x-\frac{2}{3}=-\frac{6}{7}\)

\(\Rightarrow-x=-\frac{6}{7}+\frac{2}{3}\)

\(\Rightarrow-x=-\frac{4}{21}\)

\(\Rightarrow x=\frac{4}{21}\)

\(\frac{4}{7}-x=\frac{1}{3}\)

\(\Rightarrow x=\frac{4}{7}-\frac{1}{3}\)

\(\Rightarrow x=\frac{5}{21}\)

1. -4/9 + ( -5/6) -17/4

     = -23/18 - 17/4 = -199/36

  2.tìm x

   a. -x - 2/3 = -6/7

       -x = -6/7 +2/3

       -x = -4/21

        x=4/21

   b. 4/7 - x = 1/3

          x = 4/7 - 1/3

          x =5/21 

j mà hjhj thế nguyenthuy

j mà j mà hjhj thế nguyenthuy thế trieu dang