\(3^{x+2}-3^x=72\)

\(\Rightarrow3^x\cdot\left(3^2-1\right)=72\)

\(\Rightarrow3^x\cdot\left(9-1\right)=72\)

\(\Rightarrow3^x\cdot8=72\)

\(\Rightarrow3^x=\dfrac{72}{8}\)

\(\Rightarrow3^x=9\)

\(\Rightarrow3^x=3^2\)

\(\Rightarrow x=2\)

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Xem lại đề 

\(\Leftrightarrow3.3^x-\dfrac{3^x}{3}=72\)

\(\Leftrightarrow9.3^x-3^x=3.72\)

\(\Leftrightarrow8.3^x=3.8.3^2\)

\(\Leftrightarrow3^x=3^3\Rightarrow x=3\)

\(\hept{\begin{cases}3x=4y=5z\\3x-5y+2z=72\end{cases}}\Rightarrow\hept{\begin{cases}\frac{x}{\frac{1}{3}}=\frac{y}{\frac{1}{4}}=\frac{z}{\frac{1}{5}}\\3x-5y+2z=72\end{cases}}\Rightarrow\hept{\begin{cases}\frac{3x}{1}=\frac{5y}{\frac{5}{4}}=\frac{2z}{\frac{2}{5}}\\3x-5y+2z=72\end{cases}}\)

Áp dụng tính chất dãy tỉ số bằng nhau ta có :

\(\frac{3x}{1}=\frac{5y}{\frac{5}{4}}=\frac{2z}{\frac{2}{5}}=\frac{3y-5y+2z}{1-\frac{5}{4}+\frac{2}{5}}=\frac{72}{\frac{3}{20}}=480\)

\(\frac{x}{\frac{1}{3}}=480\Rightarrow x=160\)

\(\frac{y}{\frac{1}{4}}=480\Rightarrow y=120\)

\(\frac{z}{\frac{1}{5}}=480\Rightarrow z=96\)

Vậy x = 160 ; y = 120 ; z = 96

Từ \(3x=4y=5z\)\(\Rightarrow\frac{3x}{60}=\frac{4y}{60}=\frac{5z}{60}=\frac{x}{20}=\frac{y}{15}=\frac{z}{12}\)

Áp dụng tính chất của dãy tỉ số bằng nhau ta có:

\(\frac{x}{20}=\frac{y}{15}=\frac{z}{12}=\frac{3x}{60}=\frac{5y}{75}=\frac{2z}{24}=\frac{3x-5y+2z}{60-75+24}=\frac{72}{9}=8\)

\(\Rightarrow x=8.20=160\); \(y=8.15=120\); \(z=8.12=96\)

Vậy \(x=160\); \(y=120\); \(z=96\)

72-3x=5x+8

=> -5x-3x=8-72

=> (-5-3)x=-64

=> -8x=-64

=> x=-64:(-8)

=> x=8

72-3x=5x+8

=>3x+5x=72-8

=>8x=64

=>x=8

 => 72 = 5x+3x+8.

=> 64 = 8x.

=> x =8.

=>-5x-3=8-27

=>(-5-3)x=-64

=>-8x=-64

=>x=-64:(-8)

=> x= 8

72 - 3x = 5x + 8

72 - 8 = 5x + 3x

64 = 8x

=> 8x = 64

=> x = 64 : 8

=> x = 8

Vậy x = 8

Sử dụng QUY TẮC CHUYỂN VẾ.

x = 8

72-3X=5X+8

2X=72-8

2X=64

X=64:2

X=32

`(3x-12)xx6=72`

`3x-12=72:6`

`3x-12=12`

`3x=12+12`

`3x=24`

`x=8`

$3x-12=72:6$

$3x-12=12$

$3x=12+12$

$3x=24$

x=24 chiab3�

$x=8$