From: "y Mehta" <mehtayogesh@gmail.(DOT).com> Path: news.mathworks.com!newsfeed-00.mathworks.com!webcrossing Newsgroups: comp.soft-sys.matlab Subject: Angle between two vectors Message-ID: <ef5ce9c.-1@webcrossing.raydaftYaTP> Date: Mon, 9 Jul 2007 17:16:29 -0400 Lines: 12 NNTP-Posting-Host: 63.211.21.46 MIME-Version: 1.0 Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 8bit Xref: news.mathworks.com comp.soft-sys.matlab:418088 How do I find the angle between two unit vectors a and b? I know I can find cosine theta by the following formula: theta = acos(dot(a,b)); However, how do I know whether the angle is actually theta, or -theta or pi-theta or pi+theta?? Notice that the vectors are in three dimension (3d). Thanks, -YM