Đáp án:
`a. 3x^2+4x=2x`
`<=> 3x^2+4x-2x=0`
`<=> 3x^2+2x=0`
`<=> x(3x+2)=0`
`<=>`\(\left[ \begin{array}{l}x=0\\3x+2=0\end{array} \right.\)
`<=>`\(\left[ \begin{array}{l}x=0\\x=-\dfrac{2}{3}\end{array} \right.\)
Vậy `S={0;-2/3}`
`b. 25x^2-0,64=0`
`<=> (5x)^2-0,8^2=0`
`<=> (5x-0,8)(5x+0,8)=0`
`<=>`\(\left[ \begin{array}{l}5x-0,8=0\\5x+0,8=0\end{array} \right.\)
`<=>`\(\left[ \begin{array}{l}x=0,16\\x=-0,16\end{array} \right.\)
Vậy `S={+-0,16}`
`c. x^4-16x^2=0`
`<=> x^2(x^2-16)=0`
`<=>`\(\left[ \begin{array}{l}x^2=0\\x^2-16=0\end{array} \right.\)
`<=>`\(\left[ \begin{array}{l}x=0\\x=4\\x=-4\end{array} \right.\)
Vậy `S={0;+-4}`
`d. x^2+x=6`
`<=> x^2+x-6=0`
`<=> x^2+3x-2x-6=0`
`<=> x(x+3)-2(x+3)=0`
`<=> (x+3)(x-2)=0`
`<=>`\(\left[ \begin{array}{l}x+3=0\\x-2=0\end{array} \right.\)
`<=>`\(\left[ \begin{array}{l}x=-3\\x=2\end{array} \right.\)
Vậy `S={-3;2}`
`e. x^2-7x=-12`
`<=> x^2-7x+12=0`
`<=> x^2-3x-4x+12=0`
`<=> x(x-3)-4(x-3)=0`
`<=> (x-3)(x-4)=0`
`<=>`\(\left[ \begin{array}{l}x-3=0\\x-4=0\end{array} \right.\)
`<=>`\(\left[ \begin{array}{l}x=3\\x=4\end{array} \right.\)
Vậy `S={3;4}`
