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The **angle** between two lines *a*x+*b*y+*c*=0 and
*a*x+*b*y+*c*=0 is

In particular, the two lines are **parallel** when *a**b*=*a**b*,
and **perpendicular** when *a**a*=-*b**b*.

The **angle** between two lines of slopes *m* and *m* is

arctan((*m*-*m*)/(1+*m**m*)).

In particular, the two lines are **parallel** when *m*=*m*

and **perpendicular** when *m**m*=-1.

*Silvio Levy
Wed Oct 4 16:41:25 PDT 1995*

This document is excerpted from the 30th Edition of the *CRC Standard Mathematical Tables and Formulas* (CRC Press). Unauthorized duplication is forbidden.