Is this true the sum of zero and an addend is zero?

false, adding a number to 0 like 0+9 would =9. only when you multiply 0 by a number is the answer 0.

Correct.

yes

Actually 0/0 is undefined because there is no logical way to define it. In ordinary mathematics, you cannot divide by zero. The limit of x/x as x approaches 0 exists and equals 1 so you might be...

Any negative number.