$\text{Bài 2: Tìm x}$
$\text{a,}$
`3/4` `+` `1/4` `:` `x` `=` `-2/5`
`1/4` `:` `x` `=` `-2/5` `-` `3/4`
`1/4` `:` `x` `=` `-23/20`
`x` `=` `1/4` `:` `-23/20`
`x` `=` `-5/23`
$\text{Vậy}$ `x` `=` `-5/23`
----------------------------
$\text{b,}$
`|x+0,5|-12,9=0`
`|x+0,5|=0+12,9`
`|x+0,5|=12,9`
$\Rightarrow$ $\left[\begin{matrix} x+0,5=12,9\\ x+0,5=-12,9\end{matrix}\right.$
$\Rightarrow$ $\left[\begin{matrix} x=12,9-0,5\\ x=-12,9-0,5\end{matrix}\right.$
$\Rightarrow$ $\left[\begin{matrix} x=12,4\\ x=-13,4\end{matrix}\right.$
$\text{Vậy x=12,4 hoặc x=13,4}$
------------------------------
$\text{c,}$
`(2/5 -3x)^{2}=``9/25`
`(2/5 -3x)^{2}=``(3/5)^{2}`
$\Rightarrow$ `(2/5 -3x)=`$\pm$ `(3/5)`
$\text{TH1:}$ `(2/5 -3x)=``3/5`
$\Rightarrow$ `3x=``2/5` `-` `3/5`
$\Rightarrow$ `3x=``-1/5`
$\Rightarrow$ `x=``-1/5` `:` `3`
$\Rightarrow$ `x=``-1/15`
$\text{TH2:}$ `(2/5 -3x)=``-3/5`
$\Rightarrow$ `3x=``2/5` `+` `3/5`
$\Rightarrow$ `3x=``1`
$\Rightarrow$ `x=``1` `:` `3`
$\Rightarrow$ `x=``1/3`
$\text{Vậy x=}$ `-1/15` $\text{hoặc x=}$ `1/3`
-----------------------------------
$\text{d,}$
`3^{x}+3^{x+2}=810`
$\Rightarrow$`3^{x}+3^{x}.3^{2}=810`
$\Rightarrow$`3^{x}.(1+3^{2})=810`
$\Rightarrow$`3^{x}.10=810`
$\Rightarrow$`3^{x}=810:10`
$\Rightarrow$`3^{x}=81`
$\Rightarrow$`3^{x}=^{4}`
$\Rightarrow$`x=4`
$\text{Vậy x= 4}$
