I'm sorry but your example is correct.
when A=45 and B=3 -> c=10.6256898
In the original equation the left part is 31.3747102; the right part is 42-10.6256898 -> which is exactly 31.3747102.
look, I think that mathematicians learn that much because it is the only (maybe not...) thing that challenges them. I believe that a person should do whatever he wants, if someone like Geography (though I can't understand those pupils :)) he or she should learn it and do it all their lives...