Is it acceptable to work backwards in a "show this" problem?by serllus reuel Tags: acceptable, backwards, show this, work 

#1
Dec2413, 12:18 PM

P: 54

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? 



#2
Dec2413, 12:35 PM

P: 1,324

That's probably almost always the case because if you work backwards, there's far more room for errors and you may potentially find yourself with the "wrong start" if you know what I mean. Take for example something like Hess' law. Imagine trying to work the target equation backwards to find the 'x' many given equations and molar enthalpies. That's definitely harder than using the x many equations to find the target equation. 



#3
Dec2413, 12:37 PM

PF Gold
P: 366

If you can work it backwards first you then should be able to then show it forwards. If they are just asking to show it is solution, plug it in I say!




#4
Dec3013, 07:44 AM

P: 2

Is it acceptable to work backwards in a "show this" problem?
I find it that it is easy to plug the variables into the equation. For example if you take the basic equation d=st, then rather than thinking in your brain backwards about numbers, plugging in is a lot easier.




#5
Dec3013, 02:02 PM

Mentor
P: 21,012





#6
Dec3113, 07:57 AM

Math
Emeritus
Sci Advisor
Thanks
PF Gold
P: 38,885

This is sometimes called a "synthetic" proof: you start from the conclusion and work backwards to the hypothesis. As long as it is clear that every step is reversible that's a valid proof because we could go from hypothesis to conclusion by reversing each step.



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