Is it acceptable to work backwards in a "show this" problem? In problems that ask you to "show" something (e.g. "show that the formula/equation for ____ is _____") , it it sufficient to simply justify the answer they give (working backwards to literally "show it"), or should one derive the formula, as if the answer were not there? I know this depends on the problem, for example, an exercise that asks you show that a particular solution to an ODE is correct probably does not want you to solve the ODE. There are also cases in which it is impossible to work backwards. But, what is the general rule to these problems, if any?