Hai đường thẳng \(y=ax+b\left(a\ne0\right)\) và \(y=a'x+b'\left(a'\ne0\right)\) song song nhau khi
\(\left\{{}\begin{matrix}a\ne a'\\b\ne b'\end{matrix}\right.\)\(\left\{{}\begin{matrix}a=a'\\b=b'\end{matrix}\right.\)\(aa'=-1\)\(\left\{{}\begin{matrix}a=a'\\b\ne b'\end{matrix}\right.\)