Збир углова у троуглу — разлика између измена

* {{Cite book |ref= harv|title=The Language of Mathematics: Making the Invisible Visible |last= Devlin|first=Keith J.|url=http://books.google.com/books?id=pT0mlWWgpzQC&pg=PA161|isbnid=ISBN 0-8050-7254-3 |year=2000 |publisher=Macmillan}}

* {{Cite book |ref= harv|title=CRC concise encyclopedia of mathematics |last= Weisstein|first=Eric W.|url=http://books.google.com/books?id=aFDWuZZslUUC&pg=PA2147|edition=2nd |isbnid=ISBN 1-58488-347-2 |year=2003}}