x3 - 9x - 5x2 + 45 = 0
⇔ ( x3 - 5x2 ) - ( 9x - 45 ) = 0
⇔ x2( x - 5 ) - 9( x - 5 ) = 0
⇔ ( x - 5 )( x2 - 9 ) = 0
⇔ ( x - 5 )( x - 3 )( x + 3 ) = 0
⇔ x - 5 = 0 hoặc x - 3 = 0 hoặc x + 3 = 0
⇔ x = 5 hoặc x = ±3
\(x^3-9x-5x^2+45=0\)
\(x^3-5x^2-9x+45=0\)
\(x^2\left(x-5\right)-9\left(x-5\right)=0\)
\(\left(x-5\right)\left(x^2-9\right)=0\)
\(\orbr{\begin{cases}x-5=0\\x^2-9=0\end{cases}}\)
\(\orbr{\begin{cases}x=5\\x=\pm3\end{cases}}\)
