Math problems are self checking; meaning that you can solve the problem, then plug in the answer to verify. So if you arrive at an answer and then cannot verify how you arrived at the answer, you cannot prove how you obtained it. You should be able to show step by step exactly the logic for that step. It should follow a rule of mathematics and you have to show that rule or operation. All math problems work this way, from the simplest arithmetic to the most complex quantum physics problems. So you should always follow this rule.
