Too messy to type here.
That proof should be in most textbooks, if not,
Google it.
https://search.yahoo.com/search?fr=spigot-nt-gcmac&ei=utf-8&ilc=12&type=692858&p=proof%20:%20sum%20of%20angles%20in%20a%20triangle%20equals%20180
Prove that the sum of the angles of Triangle is 180°.
1 answer