Giải phương trình: 8x^2 +30x +7=0

`8x^2+30x+7=0`

`⇔8x^2+2x+28x+7=0`

`⇔2x(4x+1)+7(4x+1)=0`

`⇔(4x+1)(2x+7)=0`

`⇔`$\left[\begin{matrix} 4x+1=1\\2x+7=0\end{matrix}\right.$

`⇔`$\left[\begin{matrix} 4x=-1\\2x=-7\end{matrix}\right.$

`⇔`$\left[\begin{matrix} x=\dfrac{-1}{4}\\x=\dfrac{-7}{2}\end{matrix}\right.$

`Vậy` `S={(-1)/4;(-7)/2}`