Me on questions 10-12, then 17. explain how aswell

Hello there, 
10. 8/√3 +√7 = -2√3 +2√7

12. √5/√7-√5 = √35+5/2

17. If rectangle A is a golden rectangle then Rectangle B must be. If Rectangle B are squares of 8 and 15, then that would mean that the two sides of Rectangle B would be √64 + √225. Which √64=8 and √225=15. So yes Rectangle B is a golden rectangle.

Hope this helps! Let me know if you need anything else! :)