What is the real evidence of this Mathematics’ Theorem?

As we know, Mathematics is a branch of knowledge which concerns on the calculation of number and things that are real. Thus, all of something that is abstract seems hard to be calculated on Mathematics. Moreover, Mathematics also gives a lot of formula and theorem to all of those who learn it and there is one theorem that is very popular and raises a problem when it will be proved, the theorem says that

·         (+) x (+) = (+)

·         (-) x (+) = (-)

·         (+) x (-)= (-)

·         (-) x (-) = (+)

I am quite sure that you will say “why does it raise a problem? It can be easily be proved -2 x -2 =4,”

Ok, I know that that theorem is easily proved with number. But, how can you answer this question

“What is the real evidence in the real life which is proved the theorem above?”

Yeah maybe you will answer good person befriend with bad person will be bad person, ha-ha I have to laugh of loud if that is your answer. Why? If that is the evidence, how about if bad person befriend with bad person? Will they become a good person? Hmm…. Hard to think, though?

Ok just for information that the question above

(“What is the real evidence in the real life which is proved the theorem above?”) is asked by my literature lecturer. He said that it is too hard to mathematicians to think about number every time while the evidence of its simple theorem in the real life is hard to be found.

NOTE: I have asked this problem to my friends whose major is in Mathematics Department and unfortunately they can give me a satisfactory answer.

Well then if you have an answer, fell free to put your answer in the comment or send to ikhasbany@gmail.com. I will appreciate all of your opinions. J