`7)3x²-75=0`

`⇔3(x²-25)=0`

`⇔3(x²-5²)=0`

`⇔3(x+5)(x-5)=0`

`⇔(x+5)(x-5)=0`

`⇔`\(\left[ \begin{array}{l}x+5=0\\x-5=0\end{array} \right.\)

`⇔`\(\left[ \begin{array}{l}x=-5\\x=5\end{array} \right.\)

Vậy `x=-5` hoặc `x=5`

`8)2x²-72=0`

`⇔2(x²-36)=0`

`⇔2(x²-6²)=0`

`⇔2(x+6)(x-6)=0`

`⇔(x+6)(x-6)=0`

`⇔`\(\left[ \begin{array}{l}x+6=0\\x-6=0\end{array} \right.\)

`⇔`\(\left[ \begin{array}{l}x=-6\\x=6\end{array} \right.\)

Vậy `x=-6` hoặc `x=6`

 `9)2x²-98=0`

`⇔2(x²-49)=0`

`⇔2(x²-7²)=0`

`⇔2(x+7)(x-7)=0`

`⇔(x+7)(x-7)=0`

`⇔`\(\left[ \begin{array}{l}x+7=0\\x-7=0\end{array} \right.\)

`⇔`\(\left[ \begin{array}{l}x=-7\\x=7\end{array} \right.\)

Vậy `x=-7` hoặc `x=7`

`10)2x²-162=0`

`⇔2(x²-81)=0`

`⇔2(x²-9²)=0`

`⇔2(x+9)(x-9)=0`

`⇔(x+9)(x-9)=0`

`⇔`\(\left[ \begin{array}{l}x+9=0\\x-9=0\end{array} \right.\)

`⇔`\(\left[ \begin{array}{l}x=-9\\x=9\end{array} \right.\)

Vậy `x=-9` hoặc `x=9`

`11)(x+3)²=4`

`⇔(x+3)²-4=0`

`⇔(x+3)²-2²=0`

`⇔(x+3+2)(x+3-2)=0`

`⇔(x+5)(x+1)=0`

`⇔`\(\left[ \begin{array}{l}x+5=0\\x+1=0\end{array} \right.\)

`⇔`\(\left[ \begin{array}{l}x=-5\\x=-1\end{array} \right.\)

Vậy `x=-5` hoặc `x=-1`

`13)(x-7)²=36`

`⇔(x-7)²-36=0`

`⇔(x-7)²-6²=0`

`⇔(x-7+6)(x-7-6)=0`

`⇔(x-1)(x-13)=0`

`⇔`\(\left[ \begin{array}{l}x-1=0\\x-13=0\end{array} \right.\)

`⇔`\(\left[ \begin{array}{l}x=1\\x=13\end{array} \right.\)

Vậy `x=1` hoặc `x=13`

`17)x²-14x+49=0`

`⇔x²-2.x.7+7²=0`

`⇔(x-7)²=0`

`⇔x-7=0`

`⇔x=7`

Vậy `x=7`

`28)4x²-4x-3=0`

`⇔4x²-6x+2x-3=0`

`⇔2x(2x-3)+(2x-3)=0`

`⇔(2x-3)(2x+1)=0`

`⇔`\(\left[ \begin{array}{l}2x-3=0\\2x+1=0\end{array} \right.\)

`⇔`\(\left[ \begin{array}{l}2x=3\\2x=-1\end{array} \right.\)

`⇔`\(\left[ \begin{array}{l}x=\dfrac{3}{2}\\x=-\dfrac{1}{2}\end{array} \right.\)

Vậy `x=3/2` hoặc `x=-1/2`

`31)x³+3x²+3x+1=0`

`⇔x³+3.x².1+3.x.1²+1³=0`

`⇔(x+1)³=0`

`⇔x+1=0`

`⇔x=-1`

Vậy `x=-1`

`32)x³-3x²+3x-1=0`

`⇔x³-3.x².1+3.x.1²-1³=0`

`⇔(x-1)³=0`

`⇔x-1=0`

`⇔x=1`

Vậy `x=1`

`34)x³-6x²+12x-8=0`

`⇔x³-3.x².2+3.x.2²-2³=0`

`⇔(x-2)³=0`

`⇔x-2=0`

`⇔x=2`

Vậy `x=2`

$\begin{array}{l} 7)3{x^2} - 75 = 3\left( {{x^2} - 25} \right) = 3\left( {x - 5} \right)\left( {x + 5} \right) = 0\\  \Leftrightarrow \left[ \begin{array}{l} x =  - 5\\ x = 5 \end{array} \right.\\ 8)2{x^2} - 72 = 2\left( {{x^2} - 36} \right) = 2\left( {x - 6} \right)\left( {x + 6} \right) = 0\\  \Leftrightarrow \left[ \begin{array}{l} x = 6\\ x =  - 6 \end{array} \right.\\ 9)2{x^2} - 98 = 2\left( {{x^2} - 49} \right) = 2\left( {x - 7} \right)\left( {x + 7} \right) = 0\\  \Leftrightarrow \left[ \begin{array}{l} x = 7\\ x =  - 7 \end{array} \right.\\ 10)2{x^2} - 162 = 2\left( {{x^2} - 81} \right) = 2\left( {x - 9} \right)\left( {x + 9} \right) = 0\\  \Leftrightarrow \left[ \begin{array}{l} x = 9\\ x =  - 9 \end{array} \right.\\ 11){\left( {x + 3} \right)^2} = 4 = {2^2}\\  \Leftrightarrow \left[ \begin{array}{l} x + 3 = 2\\ x + 3 =  - 2 \end{array} \right. \Leftrightarrow \left[ \begin{array}{l} x =  - 1\\ x =  - 5 \end{array} \right.\\ 13){\left( {x - 7} \right)^2} = 36\\  \Leftrightarrow \left[ \begin{array}{l} x - 7 = 6\\ x - 7 =  - 6 \end{array} \right. \Leftrightarrow \left[ \begin{array}{l} x = 13\\ x = 1 \end{array} \right.\\ 17){x^2} - 14x + 49 = 0\\  \Leftrightarrow {\left( {x - 7} \right)^2} = 0\\  \Leftrightarrow x = 7\\ 28)4{x^2} - 4x - 3 = 0\\  \Leftrightarrow {\left( {2x - 1} \right)^2} - 4 = 0\\  \Leftrightarrow \left( {2x - 1 - 2} \right)\left( {2x - 1 + 2} \right) = 0\\  \Leftrightarrow \left( {2x - 3} \right)\left( {2x + 1} \right) = 0\\  \Leftrightarrow \left[ \begin{array}{l} x = \dfrac{3}{2}\\ x =  - \dfrac{1}{2} \end{array} \right.\\ 31){x^3} + 3{x^2} + 3x + 1 = 0\\  \Leftrightarrow {\left( {x + 1} \right)^3} = 0 \Leftrightarrow x + 1 = 0 \Leftrightarrow x =  - 1\\ 32){x^3} - 3{x^2} + 3x - 1 = 0\\  \Leftrightarrow {\left( {x - 1} \right)^3} = 0 \Leftrightarrow x - 1 = 0 \Leftrightarrow x = 1\\ 34){x^3} - 6{x^2} + 12x - 8 = 0\\  \Leftrightarrow {\left( {x - 2} \right)^3} = 0\\  \Leftrightarrow x - 2 = 0 \Leftrightarrow x = 2 \end{array}$