I was told that when undoing operations in an equation you should start by following the PEDMAS rules, but in reverse. So, for example:

8x + 16/x = 4x + 16

According to the advice, I should start this problem by subtracting 16/x, since addition is the first operation to perform if your following PEDMAS backwards. However, online solvers of this problem perform multiplication first (by multiplying everything on both sides by x) in order to undo the division. I have seen similar problems solved in the same way, with the undoing of division being the first operation. My question is when is it your suppose to apply the reverse PEDMAS rule? Or does that rule really matter?

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    $\begingroup$ IMO, there is no such rule to be followed strictly... In order to "solve" and expression you have to perform suitable operations in order to transform it into $ax=b$ , from which $x = \dfrac b a$. $\endgroup$ Commented Apr 13, 2019 at 13:20
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    $\begingroup$ This is the reason of the first step above: multiply both sides by $x$, in order to remove the $x$ in $\dfrac {16}{x}$. $\endgroup$ Commented Apr 13, 2019 at 13:20
  • $\begingroup$ What about an equation like: 2x + 2 = 8. In this equation you would get a different answer if you tried dividing the 2x first instead of subtracting the 2. Is it just that you need to recognize the certain equations where the reverse PEDMAS rule needs to be implemented? $\endgroup$
    – Jason
    Commented Apr 13, 2019 at 13:46
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    $\begingroup$ See the post Rules for transforming equations. $\endgroup$ Commented Apr 13, 2019 at 14:15
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    $\begingroup$ There is only one rule : if $A=B$, then $A+a=B+a$ and $A \times a = B \times a$. Basically you have only to apply it (with suitable choice of $a$). $\endgroup$ Commented Apr 13, 2019 at 14:18

1 Answer 1


Subtraction first makes sense, but not for the reason you think.

Use subtraction first to collect terms on one side and simplify, so: $$8x+\frac{16}{x}-4x-16=0$$ $$\to 4x-16+\frac{16}{x}=0$$ Then multiply to undo the division, so: $$4x^2-16x+16=0$$

Then we can solve by factorising.


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