a) \(\frac{2}{3}.3^{x+1}-7.3^x=-405\)

=> \(\frac{2}{3}.3^x.3-7.3^x=-405\)

=> \(2.3^x-7.3^x=-405\)

=> \(3^x\left(2-7\right)=-405\)

=> \(3^x.\left(-5\right)=-405\)

=> \(3^x=-405:\left(-5\right)\)

=> \(3^x=81\)

=> \(x=4\)

b) \(\frac{3}{1-2x}=\frac{-5}{3x-2}\)

=> \(3\left(3x-2\right)=-5\left(1-2x\right)\)

=> \(9x-6=-5+10x\)

=> \(9x-10x=-5+6\)

=> \(-x=1\)

=> \(x=-1\)

2x = 3y => 10x=15y
5y = 7z => 15y=21z
=> 10x=15y=21z =>x=2,1z
y=1,4z
Mà : 3x - 7y + 5z = 30 => 6,3z - 9,8z + 5z=30 =>1,5z=30
=>z=20
y=28
x=42

Từ \(2x=3y\)\(\Rightarrow\frac{x}{3}=\frac{y}{2}=\frac{x}{3}.\frac{1}{7}=\frac{y}{2}.\frac{1}{7}=\frac{x}{21}=\frac{y}{14}\)( 1 )

Từ \(5y=7z\)\(\Rightarrow\)\(\frac{y}{7}=\frac{z}{5}=\frac{y}{7}.\frac{1}{2}=\frac{z}{5}.\frac{1}{2}=\frac{y}{14}=\frac{z}{10}\)( 2 )

Từ ( 1 ) và ( 2 ) \(\Rightarrow\)\(\frac{x}{21}=\frac{y}{14}=\frac{z}{10}\)

Đặt \(\frac{x}{21}=\frac{y}{14}=\frac{z}{10}=k\)

\(\Rightarrow\hept{\begin{cases}x=21k\\y=14k\\z=10k\end{cases}}\)

Thay vào \(3x+5z-7y=30\)ta có ;

\(3.21k+5.10k-7.14k=30\)

\(63k+50k-98k=30\)

\(15k=30\)

\(k=2\)

Thay vào ta được :

\(\Rightarrow\hept{\begin{cases}x=21.2\\y=14.2\\z=10.2\end{cases}}\)\(\Rightarrow\hept{\begin{cases}x=42\\y=28\\z=20\end{cases}}\)

cac ban giup do minh nhe cm on ban nao da giup minh giai bai nay vi minh do mon toan lam

a,\(x+\frac{5}{4}=\frac{3}{2}\)

\(x=\frac{3}{2}-\frac{5}{4}\)

\(x=\frac{1}{4}\)

b,/ x + 2/ + 2,1 =5,7

   / x + 2/         =5,7 - 2,1

   / x + 2/         =3,6

   \(x+2=\orbr{\begin{cases}-3,6\Rightarrow x=-3,6-2=-5,6\\3,6\Rightarrow x=3,6-2=1,6\end{cases}}\)

Xin lỗi, mình không viết cách viết dấu giá trị tuyệt đối

c,\(\frac{x}{3}=\frac{2}{15}\)

\(\Rightarrow x.15=3.2\)

\(\Rightarrow x.15=6\)

\(\Rightarrow x=6:15\)

\(\Rightarrow x=\frac{2}{5}\)

d,7,2x + ( -3,7x ) - 2,7 = 7,8

\(\Rightarrow7,2x+\left(-3,7x\right)=7,8+2,7\)

\(\Rightarrow7,2x+\left(-3,7x\right)=10,5\)

\(\Rightarrow x\left(7,2+\left(-3,7\right)\right)=10,5\)

\(\Rightarrow3,5x=10,5\)

\(\Rightarrow x=10,5:3,5\)

\(\Rightarrow x=3\)