http://z5lcip4dafatwwa6hvyibizpzwycvwp67cjga3hzjhxhwvuyaqavxnid.onion/math/497
Referenced by: P935 P935 Wed 2022-05-18 21:42:16 link reply P934 Not that anon, but I would usually explain it as part of the definition of what it means to multiply two integers together. Multiplying by a natural number is repeated addition, and multiplying by a negative integer is multiplying by a natural number, then flipping the sign. Alternately, we could define multiplication by a negative integer as repeated subtraction, starting at 0.