Lời giải:
a. $\frac{a}{b}=\frac{3}{4}\Rightarrow \frac{a}{3}=\frac{b}{4}$

Đặt $\frac{a}{3}=\frac{b}{4}=k\Rightarrow a=3k; b=4k$ 

Khi đó:
$B=\frac{2a-5b}{a-3b}=\frac{2.3k-5.4k}{3k-3.4k}=\frac{-14k}{-9k}=\frac{14}{9}$

b.

$a-b=7\Rightarrow b=a-7$. Khi đó:
$C=\frac{3a-b}{2a+7}+\frac{3b-a}{2b-7}=\frac{3a-(a-7)}{2a+7}+\frac{2b+a-7-a}{2b-7}$
$=\frac{2a+7}{2a+7}+\frac{2b-7}{2b-7}=1+1=2$

Lời giải:
a. $(3x-\frac{1}{5})^3=\frac{64}{125}=(\frac{4}{5})^3$

$3x-\frac{1}{5}=\frac{4}{5}$

$3x=\frac{4}{5}+\frac{1}{5}=1$

$x=\frac{1}{3}$

b/

$(2x+\frac{3}{7})^2=\frac{100}{49}=(\frac{10}{7})^2=(\frac{-10}{7})^2$

$\Rightarrow 2x+\frac{3}{7}=\frac{10}{7}$ hoặc $2x+\frac{3}{7}=\frac{-10}{7}$

$\Rightarrow x=\frac{1}{2}$ hoặc $x=\frac{-13}{14}$

c.

$(x+\frac{2}{9})^5=(x+\frac{2}{9})^3$

$(x+\frac{2}{9})^5-(x+\frac{2}{9})^3=0$

$(x+\frac{2}{9})^3[(x+\frac{2}{9})^2-1]=0$

$\Rightarrow (x+\frac{2}{9})^3=0$ hoặc $(x+\frac{2}{9})^2=1$

Nếu $(x+\frac{2}{9})^3=0\Leftrightarrow x=\frac{-2}{9}$

Nếu $(x+\frac{2}{9})^2=1=1^2=(-1)^2$

$\Rightarrow x+\frac{2}{9}=1$ hoặc $x+\frac{2}{9}=-1$

$\Rightarrow x=\frac{7}{9}$ hoặc $x=\frac{-11}{9}$

bỏ chữ `d)` ra nhé

các bạn giúp mình được không ạ??

các pạn giúp mk nhanh dc ko.

gọi số bút công mua là x đk x >0

theo bài ra ta có ( x - 8): 3 = x -16⇔

                              x-8 = 3x - 48

                     ⇔ 3x - x =  48 - 8

                         ⇔ 2x =   40

                             x = 40 : 2

                           x = 20

tất cả số bút công mua có giá là 3500 x 20 = 70000(đồng)

đs....

`@`\(\dfrac{13}{4}+4x=\dfrac{7}{9}\)

`=>4x=-89/36`

`=>x=-89/144`

`@`\(\dfrac{5}{3}.\left(x-1\right)^3=\left(-2\right)^3\)

`=>5/3 .(x-1)^3=-8`

`=>(x-1)^3=-24/5`

\(\Rightarrow x-1=\sqrt[3]{-\dfrac{24}{5}}\)

\(\Rightarrow x=\sqrt[3]{-\dfrac{24}{5}}+1\)

`@`\(\left|2x+1\right|-1^2=3^2\)

\(\Rightarrow\left|2x+1\right|=10\)

\(\Rightarrow\left[{}\begin{matrix}2x+1=10\\2x+1=-10\end{matrix}\right.\) \(\Rightarrow\left[{}\begin{matrix}x=\dfrac{9}{2}\\x=-\dfrac{11}{2}\end{matrix}\right.\)

\(0,5x+\dfrac{1}{3}-\dfrac{2}{3}\left(x+1\right)=\dfrac{2}{9}\)

\(\dfrac{1}{2}x+\dfrac{1}{3}-\dfrac{2}{3}x-\dfrac{2}{3}=\dfrac{2}{9}\)

\(\dfrac{-1}{6}x-\dfrac{1}{3}=\dfrac{2}{9}\)

\(\dfrac{-1}{6}x=\dfrac{5}{9}\)

\(x=\dfrac{5}{9}:\dfrac{-1}{6}\)

\(x=-\dfrac{10}{3}\)

     2¹⁵x 9⁴ : 6⁶ x 8³ 

 = 2¹⁵ x (  3²)⁴ : (2x3)⁶ x (2³)³

= 2¹⁵ x 3⁸  : (2⁶ x3⁶) x 2⁹

= 2¹⁸ x 3² 

\(\dfrac{2^{15}.9^4}{6^6.8^3}=\dfrac{2^{15}.3^8}{2^6.3^6.2^9}=\dfrac{2^{15}}{2^{6+9}}.\dfrac{3^8}{3^6}=3^2=9\)

\(D=-20\%:\left(-\dfrac{1}{2}\right)^2-\left(-1\right)^{2021}.2\dfrac{1}{3}-0,2\)

\(\Rightarrow D=-\dfrac{1}{5}:\dfrac{1}{4}+1.\dfrac{7}{3}-\dfrac{1}{5}\)

\(\Rightarrow D=-\dfrac{4}{5}+\dfrac{7}{3}-\dfrac{1}{5}\)

\(\Rightarrow D=-1+\dfrac{7}{3}=\dfrac{4}{3}\)

Trả lời nhanh giúp mik ạ