Because 0 is not 1. Consider the equation x = 5/y. Let us assume that the variable x is defined for every value of y. Then x * y = 5 (after multiplying both si

Because 0 is not 1. Consider the equation x = 5/y. Let us assume that the variable x is defined for every value of y. Then x * y = 5 (after multiplying both sides by y). Let us also assume, as you suggest, that when y = 0, x = 5/0 = 5. Then x = 5 = x * y. The only solution to this equation is y = 1, a contradiction.