aops, https://artofproblemsolving.com/community/c6h3617076p35436137

 

FB https://www.facebook.com/photo.php?fbid=10162825605139371&set=p.10162825605139371&type=3

 

Aops, https://artofproblemsolving.com/community/c4t48f4h3616471_find_the_value_of_angle_c

 

Problem, lost source

 

Another solution of https://www.facebook.com/photo?fbid=4695212660554761&set=gm.1282324112279677

 

https://www.facebook.com/photo/?fbid=10162011604082198&set=gm.1458909885546321&idorvanity=1216680106435968

 

FB page of Nelson Tunala, https://www.facebook.com/photo/?fbid=23905833795725106&set=a.373249732743510

 

Fatih Sultan Yildirim Mehmet, I Love Geometry, https://www.facebook.com/photo/?fbid=10164331095994617&set=gm.1456142312489745&idorvanity=1216680106435968

 

I love Geometry, https://www.facebook.com/photo?fbid=24066772916296920&set=gm.1456071139163529&idorvanity=1216680106435968

 

https://www.facebook.com/photo?fbid=7897833440299617&set=a.712949622121404

 

Thiago Durado, https://www.facebook.com/photo?fbid=1023727319928226&set=gm.1433149611369642&idorvanity=1193487768669162

 

FB page of Chris Wildhagen, https://www.facebook.com/photo?fbid=1481372766356568&set=gm.1991861554917537&idorvanity=830782544358783

 

Mathstackexchange, https://math.stackexchange.com/questions/4112110/need-a-pure-geometric-solution-to-a-20-30-130-triangle-question?rq=1