a,\(x^2-4x-5=0\)
\(\Rightarrow x^2-x+5x-5=0\)
\(\Rightarrow x\left(x-1\right)+5\left(x-1\right)=0\)
\(\Rightarrow\left(x-1\right)\left(x+5\right)=0\Rightarrow\left[{}\begin{matrix}x-1=0\\x+5=0\end{matrix}\right.\)
\(\Rightarrow\left[{}\begin{matrix}x=1\\x=-5\end{matrix}\right.\)
b, \(2x^2-6x+4=0\)
\(\Rightarrow2x^2-2x-4x+4=0\)
\(\Rightarrow2x\left(x-1\right)-4\left(x-1\right)=0\)
\(\Rightarrow\left(x-1\right)\left(2x-4\right)=0\Rightarrow\left[{}\begin{matrix}x-1=0\\2x-4=0\end{matrix}\right.\)
\(\Rightarrow\left[{}\begin{matrix}x=1\\x=2\end{matrix}\right.\)
c, \(5x^2-x-18=0\)
\(\Rightarrow5x^2-10x+9x-18=0\)
\(\Rightarrow5x\left(x-2\right)+9\left(x-2\right)=0\)
\(\Rightarrow\left(x-2\right)\left(5x+9\right)=0\Rightarrow\left[{}\begin{matrix}x-2=0\\5x+9=0\end{matrix}\right.\)
\(\Rightarrow\left[{}\begin{matrix}x=2\\x=-\dfrac{9}{5}\end{matrix}\right.\)
d, \(2x^3-3x^2+x+30\)
(bạn xem lại đề nha)
e, \(5x^3-21x^2+11x+5\)
\(=5x^3-5x^2-16x^2+16x-5x+5\)
\(=5x^2\left(x-1\right)-16x\left(x-1\right)-5\left(x-1\right)\)
\(=\left(x-1\right)\left(5x^2-16x-5\right)\)
Chúc bạn học tốt!!!
