a: \(\Leftrightarrow\left(x+4\right)\left(x+5\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}x=-4\\x=-5\end{matrix}\right.\)
\(a,x^2+9x+20=0\\ \Leftrightarrow\left(x^2+4x\right)+\left(5x+20\right)=0\\ \Leftrightarrow x\left(x+4\right)+5\left(x+4\right)=0\\ \Leftrightarrow\left(x+4\right)\left(x+5\right)=0\\ \Leftrightarrow\left[{}\begin{matrix}x=-4\\x=-5\end{matrix}\right.\)
\(b,x^2+x-2=0\\ \Leftrightarrow\left(x^2+2x\right)-\left(x+2\right)=0\\ \Leftrightarrow x\left(x+2\right)-\left(x+2\right)=0\\ \Leftrightarrow\left(x-1\right)\left(x+2\right)=0\\ \Leftrightarrow\left[{}\begin{matrix}x=1\\x=-2\end{matrix}\right.\)
