Constructing an Angle of 90 degrees

How to construct an angle of 90° (using just a straightedge and compass, without a protractor)? Find the answer in this video. Construction on paper. Video without words. My math materials in English: http://www.antonija-horvatek.from.hr/materials-English.htm . My (whole) website: http://www.antonija-horvatek.from.hr/ . Thanks to Rex Boggs, Rockhampton Grammar School, Australia (http://bit.ly/rexboggs ) for help with the English translation. Music by longzijun, http://longzijun.wordpress.com/ . Terms of use allow for free use in monetized YouTube videos: https://longzijun.wordpress.com/music/free-background-music-series-terms-of-use/ .

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