Question

x² - 2x -15=0

 

Answer 1

=x^2-5x+3x-15

=x(x-5)+3(x-5)

=(x-5)(x+3)

Answer 2

\(x^2-2x-15=0\)

\(\Leftrightarrow x^2-5x+3x-15=0\)

\(\Leftrightarrow x\left(x-5\right)+3\left(x-5\right)=0\)

\(\Leftrightarrow\left(x-5\right)\left(x-3\right)=0\)

\(\Leftrightarrow\left[{}\begin{matrix}x-5=0\\x+3=0\end{matrix}\right.\)

\(\Leftrightarrow\left[{}\begin{matrix}x=5\\x=-3\end{matrix}\right.\)

Vậy: \(S=\left\{5;-3\right\}\)