Tìm x, biết: a) 5x(x-3)-x^2+9=0 b) x+4x^2+4x^3 c)(x^2-4)/(x+1)(x-2)

a) $5x(x-3)-x^2+9=0$ 

$\iff 5x(x-3)-(x^2-9)=0$                           

$\iff 5x(x-3)-(x-3)(x+3)=0$

$\iff (x-3)(5x-x-3)=0$

$\iff (x-3)(4x-3)=0$

$\iff [\begin{array}{l}x-3=0\\ 4x-3=0\end{array}$

$\iff [\begin{array}{l}x=3\\ x=\frac{3}{4}\end{array}$

b) $x+4x^2+4x^3=0$

$\iff x(1+4x+4x^2)=0$

$\iff x(4x^2+4x+1)=0$

$\iff x{(2x+1)}^2=0$

$\iff [\begin{array}{l}x=0\\ {(2x+1)}^2=0\end{array}$

$\iff [\begin{array}{l}x=0\\ 2x+1=0\end{array}$