text: why minus * minus is plus? dont tell tat its a rule. i heard tat we can find the answer using "number line". but i couldnt get enough explanation. i will prove why (-1)*(-1) = +1. the logic can be extended to all numbers.  (-1)*(-1) = (-1)*(-1) + 0           = (-1)*(-1) + 1 -1           = 1 + [(-1)*(-1) + (-1)]  take (-1) common. (-1)*(+1)=(-1) was proved by euler; so using that;             = 1 + (-1)[-1 + 1]           = 1 + (-1)*0           = 1  thus; (-1)*(-1)  = + 1