Đáp án:
`a)x∈{0;-2/7}`
`b)S={9;-5/2}`
`c)x=4`
`d)x=-3/2`
Giải thích các bước giải:
`a)7x²+2x=0`
`⇔x(7x+2)=0`
`⇔`$\left[\begin{matrix} x=0\\ 7x+2=0\end{matrix}\right.$
`⇔`$\left[\begin{matrix} x=0\\ x=-\dfrac{2}{7}\end{matrix}\right.$
Vậy `x∈{0;-2/7}`
`b)2x(x-9)+5(x-9)=0`
`⇔(x-9)(2x+5)=0`
`⇔`$\left[\begin{matrix} x-9=0\\ 2x+5=0\end{matrix}\right.$
`⇔`$\left[\begin{matrix} x=9\\ x=-\dfrac{5}{2}\end{matrix}\right.$
Vậy `S={9;-5/2}`
`c)x²-8x+16=0`
`⇔x²-2.x.4+4²=0`
`⇔(x-4)²=0`
`⇔x-4=0`
`⇔x=4`
Vậy `x=4`
`d)4x²+12x+9=0`
`⇔(2x)²+2.2x.3+3²=0`
`⇔(2x+3)²=0`
`⇔2x+3=0`
`⇔2x=-3`
`⇔x=-3/2`
Vậy `x=-3/2`
