https://artofproblemsolving.com/wiki/index.php/AMC_10_Problems_and_Solutions

https://youtu.be/5cP_BA5JeRE. ptolemy theorem

dot product vs cross product

https://www.mathsisfun.com/algebra/vectors-dot-product.html

https://www.mathsisfun.com/algebra/vectors-cross-product.html

amc 10 problems

https://www.mymathcounts.com/Forum/index.php?topic=3360.0

https://sites.google.com/site/hhsmathcontests/home/amc-10-practice-problems

http://billingswest.billings.k12.mt.us/math/amc.htm

http://www.ltisdschools.org/page/2743

http://www.bcsd.org/webpages/bmunch/amc.cfm?subpage=46353

http://fac-staff.seattleu.edu/nirmalag/web/MPMathclub/AMC10Group5.html