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=https://books.google.com/books?id=pT0mlWWgpzQC&pg=PA161|id=ISBN 978-0-8050-7254-39 |year=2000 |publisher=Macmillan}}

* {{Cite book |ref= harv|title=CRC concise encyclopedia of mathematics |last= Weisstein|first=Eric W.|url=https://books.google.com/books?id=aFDWuZZslUUC&pg=PA2147|edition=2nd |id=ISBN 978-1-58488-347-20 |year=2003}}