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Three lines *a*x+*b*y+*c*=0,
*a*x+*b*y+*c*=0, and *a*x+*b*y+*c*=0 are **concurrent** if and
only if

(This remains true in **oblique coordinates**.)

Three points (*x*,*y*), (*x*,*y*) and (*x*,*y*) are
**collinear** if and only if

(This remains true in **oblique coordinates**.)

Three points with polar coordinates
(*r*,), (*r*,) and (*r*,) are
collinear if and only if

*r**r* sin(-)+
*r**r* sin(-)+
*r**r* sin(-)=0.

*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.