text: I will prove why (-1)*(-1) = +1. The logic can be extended to all numbers.\n\n(-1)*(-1) = (-1)*(-1) + 0\n          = (-1)*(-1) + 1 -1\n          = 1 + [(-1)*(-1) + (-1)]\n\nTake (-1) common. (-1)*(+1)=(-1) was proved by Euler; so using that;\n \n          = 1 + (-1)[-1 + 1]\n          = 1 + (-1)*0\n          = 1\n\nThus; (-1)*(-1)  = + 1