a,5x(x-1)+x(x+17)=0
⇔ 5x² - 5x + x² + 17x = 0
⇔ 6x² + 12x = 0
⇔ 6x(x + 2) = 0
⇔ \(\left[ \begin{array}{l}x=0\\x=-2\end{array} \right.\)
Vậy x ∈ { 0 ; - 2 }
b,x(5x^2+6)-x(4x^2+6)=0
⇔ 5x³ + 6x - 4x³ - 6x = 0
⇔ 4x³ = 0
⇔ x = 0
c,3x(x-3)²-3x(x+3)²=0
⇔ 3x [ ( x - 3 )² - ( x + 3 )² ] = 0
⇔ 3x (x - 3 + x + 3 )(x - 3 - x - 3 ) = 0
⇔ 3x . 2x . ( - 6 ) = 0
⇔ - 36x² = 0
⇔ x = 0