斜率者，線之正切也，亦線之高除以進，記曰k，有
k = tan α = y 2 − y 1 x 2 − x 1 = y 1 − y 2 x 1 − x 2 {\displaystyle k=\tan \alpha ={\frac {y_{2}-y_{1}}{x_{2}-x_{1}}}={\frac {y_{1}-y_{2}}{x_{1}-x_{2}}}}
