凡坐標幾何之直線，盡可述以下式。
y − y 0 = k ( x − x 0 ) {\displaystyle y-y_{0}=k(x-x_{0})}
令點斜式過(x0,y0)=(0,b)，得
y = k x + b {\displaystyle y=kx+b}
亦屬一次函數之式
y − y 1 y 2 − y 1 = x − x 1 x 2 − x 1 {\displaystyle {\frac {y-y_{1}}{y_{2}-y_{1}}}={\frac {x-x_{1}}{x_{2}-x_{1}}}}
y b + x a = 1 {\displaystyle {\frac {y}{b}}+{\frac {x}{a}}=1}
A x + B y + C = 0 {\displaystyle Ax+By+C=0}
