The angle bisectors of lines L1: a1x + b1y + c1 = 0 and L2: a2x + b2y + c2 = 0 are given by $\frac{a1x + b1y + c1}{sqrt}$($a1^2$ + $b1^2$) = +/-$\frac{a2x + b2y + c2}{sqrt}$($a2^2$ + $b2^2$). The positive sign gives one bisector and the negative sign gives the other. To identify which is the acute angle bisector: first make c1 and c2 the same sign, then check a1a2 + b1b2. If this product is negative, the positive sign gives the acute bisector. If positive, the negative sign gives the acute bisector. The bisector of the angle containing the origin is identified by making c1 and c2 positive and taking the positive sign. These bisectors are perpendicular to each other.