Off by one character

There was a discussion on Twitter today about a mistake calculus students make:

\frac{d}{dx}e^x = x e^{x-1}

I pointed out that it’s only off by one character:

\frac{d}{de}e^x = x e^{x-1}

The first equation is simply wrong. The second is correct, but a gross violation of convention, using x as a constant and e as a variable.

 

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