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The angle between two lines ax+by+c=0 and ax+by+c=0 is
In particular, the two lines are parallel when ab=ab, and perpendicular when aa=-bb.
The angle between two lines of slopes m and m is
In particular, the two lines are parallel when m=m
and perpendicular when mm=-1.
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.