Auxiliary line

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An auxiliary line is an extra line needed to complete a proof in plane geometry.[1] For example, to prove that the sum of the angles of a triangle is 180 degrees, it is necessary to draw an auxiliary line.


  1. ^ Leff, Lawrence S. (1997). Geometry the Easy Way (Barron's E–Z). Barron's Educational Series; 3rd edition. p. 107. ISBN 978-0764101106.