$\text{Bài 3 :}$
`(2x-1)^8=(2x-1)^6`
`⇒(2x-1)^8-(2x-1)^6=0`
`⇒(2x-1)^6 . [(2x-1)²-1]=0`
`⇒`\(\left[ \begin{array}{l}(2x-1)^6=0\\(2x-1)²-1=0\end{array} \right.\)
$\text{TH1 :}$ `(2x-1)^6=0`
`⇒ 2x-1=0`
`⇒ 2x=1`
`⇒ x=1/2`
$\text{TH2 :(2x-1)²-1=0 }$
`⇒ (2x-1)²=1`
`⇒`\(\left[ \begin{array}{l}2x-1=1\\2x-1=-1\end{array} \right.\)
`⇒`\(\left[ \begin{array}{l}2x=2\\2x=0\end{array} \right.\)
`⇒`\(\left[ \begin{array}{l}x=1\\x=0\end{array} \right.\)
$\text{Bài 4 :}$
`a)` `3^x + 3^(x+2)=2430`
`⇒ 3^x.(1+3²)=2430`
`⇒ 3^x .10=2430`
`⇒ 3^x=243`
`⇒ 3^x = 3^5`
`⇒ x=5`
`b)` `3^(x-1) +5.3^(x-1)=162`
`⇒ 3^(x-1) .(1+5)=162`
`⇒ 3^(x-1) . 6=162`
`⇒ 3^(x-1)= 27`
`⇒3^(x-1)=3³`
`⇒ x-1=3`
`⇒ x=4`