曲率

描述曲线弯曲程度的量，数值上为单位弧线上切线转过的角度

直角坐标系：

$$K=\frac{|y^{\prime \prime }|}{(1+y^{\prime 2}{)}^{1.5}}$$

参数方程：

$$\{\begin{array}{l}x=\phi (t)\\ y=\psi (t)\end{array}$$ $$K=\frac{\left|{\phi }^{\prime }(t){\psi }^{\prime \prime }(t)-{\phi }^{\prime \prime }(t){\psi }^{\prime }(t)\right|}{{\left[{\phi }^{\prime 2}(t)+{\psi }^{\prime 2}(t)\right]}^{3/2}}$$