a: =>(7x-15)x8=48

=>7x-15=6

=>7x=21

hay x=3

\(\left(7x-15\right)8=27+21\\ \left(7x-15\right)8=48\\ 7x-15=48:8\\ 7x-15=6\\ 7x=6+15\\ 7x=21\\ x=21:7\\ x=3\)

\(\left(7x-15\right).8-21=27\\ \Rightarrow\left(7x-15\right).8=48\\ \Rightarrow7x-15=6\\ \Rightarrow7x=21\\ \Rightarrow x=3\)

(7x - 15) = 27+21

(7x - 15)= 48

7x= 48+15

7x= 63

x= 63:7

x= 9

56x -120 -21 =27

56x                = 27+210 +21

56x                = 168

x                    = 3

\(\left(7x-15\right).8-21=27.\Leftrightarrow\left(7x-15\right).8=48.\Leftrightarrow7x-15=6.\Leftrightarrow x=3.\)

vào fx viết rõ đề

Bài 1:

\(\frac{8}{7}x-\frac{10}{3}=-\frac{5}{21}\)

\(\Rightarrow\frac{8}{7}x=\frac{5}{6}\)

\(\Rightarrow x=\frac{35}{48}\)

Bài 2:

\(\frac{x}{27}=-\frac{2}{3,6}\)

\(\Rightarrow x=\frac{\left(-2\right)\cdot27}{3,6}=-15\)

a: =1/8-1/8+2/5=2/5

b: =(-1/15-14/15)+(23/27+31/27)=2-1=1

c: \(=\dfrac{3}{7}\left(\dfrac{22}{21}+\dfrac{5}{21}+\dfrac{15}{21}\right)=\dfrac{3}{7}\cdot2=\dfrac{6}{7}\)

d: \(=\dfrac{-8}{9}\cdot\dfrac{3}{2}+\dfrac{1}{9}\cdot\dfrac{-3}{2}=-\dfrac{3}{2}\)

1.

b) \(3^x+3^{x+2}=2430\)

\(\Rightarrow3^x.1+3^x.3^2=2430\)

\(\Rightarrow3^x.\left(1+3^2\right)=2430\)

\(\Rightarrow3^x.10=2430\)

\(\Rightarrow3^x=2430:10\)

\(\Rightarrow3^x=243\)

\(\Rightarrow3^x=3^5\)

\(\Rightarrow x=5\)

Vậy \(x=5.\)

c) \(\left(2x-15\right)^5=\left(2x-15\right)^3\)

\(\Rightarrow\left(2x-15\right)^5-\left(2x-15\right)^3=0\)

\(\Rightarrow\left(2x-15\right)^3.\left[\left(2x-15\right)^2-1\right]=0\)

\(\Rightarrow\left[{}\begin{matrix}\left(2x-15\right)^3=0\\\left(2x-15\right)^2-1=0\end{matrix}\right.\Rightarrow\left[{}\begin{matrix}2x-15=0\\\left(2x-15\right)^2=1\end{matrix}\right.\Rightarrow\left[{}\begin{matrix}2x=15\\2x-15=\pm1\end{matrix}\right.\)

\(\Rightarrow\left[{}\begin{matrix}x=15:2\\2x-15=1\\2x-15=-1\end{matrix}\right.\Rightarrow\left[{}\begin{matrix}x=\frac{15}{2}\\2x=16\\2x=14\end{matrix}\right.\Rightarrow\left[{}\begin{matrix}x=\frac{15}{2}\\x=8\\x=7\end{matrix}\right.\)

Vậy \(x\in\left\{\frac{15}{2};8;7\right\}.\)

Chúc bạn học tốt!