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# Math Formulas: Lines in three dimensions

### 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:

 $$\begin{vmatrix} x_2-x_1 & y_2 - y_1 & z_2 - z_1 \\ a_1 & b_1 & c_1 \\ a_2 & b_2 & c_2 \end{vmatrix} = 0$$