Cho hai đường thẳng d1 {a_1}x + {b_1}y + {c_1} = 0 và d2: {a_2}x + {b_2}y + {c_2} = 0, với {a_2}.{b_2}.{c_2} khác 0

\(\frac{{{a_1}}}{{{a_2}}} \ne \frac{{{b_1}}}{{{b_2}}}\).

\(\frac{{{a_1}}}{{{a_2}}} = \frac{{{b_2}}}{{{b_1}}} = \frac{{{c_1}}}{{{c_2}}}\).

\[\frac{{{a_1}}}{{{a_2}}} = \frac{{{b_1}}}{{{b_2}}} = \frac{{{c_1}}}{{{c_2}}}\].

\(\frac{{{a_1}}}{{{a_2}}} = \frac{{{b_1}}}{{{b_2}}} \ne \frac{{{c_1}}}{{{c_2}}}\).