## Line forms

Point direction form: $$\frac{x-x_1}{a} = \frac{y - y_1}{b} = \frac{z-z_1}{c}$$ Two point form: $$\frac{x-x_1}{x_2-x_1} = \frac{y - y_1}{y_2-y_1} = \frac{z-z_1}{z_2-z_1}$$ Parametric form: \begin{aligned} x &= x_1 +t\,\cos \alpha \\ y &= y_1 +t\,\cos \beta \\ z &= z_1 +t\,\cos \gamma \end{aligned}

## Distance between two lines in 3 dimensions

 The distance from $$P_2(x_2,y_2,z_2)$$ to the line through $$P_1(x_1,y_1,z_1)$$ in the direction $$(a,b,c)$$ is
$$d = \sqrt{ \frac{\left[c(y_2-y_1)-b(z_2-z_1)\right]^2 + \left[a(z_2-z_1)-c(x_2-x_1)\right]^2 + \left[b(x_2-x_1)-a(y_2-y_1)\right]^2} {a^2 + b^2 + c^2 } }$$ The distance between two lines.
 First one: through $$P_1(x_1,y_1,z_1)$$ in direction $$(a_1,b_1,c_1)$$ , Second one: through $$P_2(x_2,y_2,z_2)$$ in direction $$(a_2,b_2,c_2)$$ is:
$$d = \frac{ \begin{vmatrix} x_2-x_1 & y_2-y_1 & z_2-z_1 \\ a_1 & b_1 & c_1 \\ a_2 & b_2 & c_2 \end{vmatrix} } { \sqrt{\begin{vmatrix} b_1 & c_1 \\ b_2 & c_2 \end{vmatrix}^2 + \begin{vmatrix} c_1 & a_1 \\ c_2 & a_2 \end{vmatrix}^2 + \begin{vmatrix} a_1 & b_1 \\ a_2 & b_2 \end{vmatrix}^2 }}$$ The two lines intersect if: $$\begin{vmatrix} x_2-x_1 & y_2 - y_1 & z_2 - z_1 \\ a_1 & b_1 & c_1 \\ a_2 & b_2 & c_2 \end{vmatrix} = 0$$