Question

Tìm x:

a) 2x³ - 50x=0

b) x³ - 8= (x-2)³

c) x³ + 5x² -4x -20= 0

Answer 1

a) 2x³-50x=2x(x²-25)=0

=> \(\left\{{}\begin{matrix}2x=0=>x=0\\x^2-25=0=>x=\pm5\end{matrix}\right.\)

b) x³-8=(x-2)³

<=>(x-2)(x²+2x+4)=(x-2)(x²-4x+4)

<=>(x-2)(x²+2x+4)-(x-2)(x²-4x+4)=0

<=>(x-2)(x²-x²+2x+4x+4-4)

<=>6x(x-2)=0

\(=>\left\{{}\begin{matrix}6x=0=>x=0\\x-2=0=>x=2\end{matrix}\right.\)

c) x³+5x²-4x-20=0

<=>x²(x+5)-4(x+5)=0

<=>(x²-4)(x+5)=0

\(=>\left\{{}\begin{matrix}x^2-4=0=>x=\pm2\\x+5=0=>x=-5\end{matrix}\right.\)