Đáp án:
\(a)\left[ \begin{array}{l}x=0\\x=7\end{array} \right.\) `
`b) x = -1/2`
`c) x= 7/2`
Giải thích các bước giải:
a) `-4x^2 + 28x = 0 => -4x(x - 7) = 0 =>` \(\left[ \begin{array}{l}-4x=0\\x-7=0\end{array} \right.\) `=>` \(\left[ \begin{array}{l}x=0\\x=7\end{array} \right.\)
`b) 2x^3 + 2x + x^2 + 1 = 0 => 2x(x^2 + 1) + (x^2 + 1) = 0 => (x^2 + 1)(2x + 1) = 0 => 2x + 1 = 0 => x = -1/2`
`c) 7x^2 - 16x = 2x^3 - 56 => 7x^2 - 16x - 2x^3 + 56 = 0 => 7x^2 + 56 - 2x^3 - 16x = 0` $\\$ `=> 7(x^2 + 8) - (2x^3 + 16x) = 0 => 7(x^2 + 8) - 2x(x^2 + 8) = 0` $\\$ `=> (x^2 + 8)(7 - 2x) = 0 => 7 - 2x = 0 => x = 7/2`
