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.
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/ .
My math materials in English: http://www.antonija-horvatek.from.hr/~ahorvate/materials-English.htm .
My (whole) website: http://www.antonija-horvatek.from.hr/ .
