# Can I ever go wrong if I keep thinking of derivatives as ratios?

I have been forewarned about it, I have read the answers here, but I haven’t seen a counter example where it doesn’t work. I know that it isnt really a fraction, but does it effectively get the same result in all cases, or are there counterexamples I must be warned about.

I do not mean obvious counter examples, like $$\frac{dy}{dx}+\frac{du}{dv} = \frac{dydv+dxdu}{dx dv}$$ which as far as I know doesn’t really mean anything.

#### Solutions Collecting From Web of "Can I ever go wrong if I keep thinking of derivatives as ratios?"

$\frac{dy}{dx}$ can indeed be thought of as a ratio. The question is, what do $dx$ and $dy$ represent in this ratio? The answer is, changes in $x$ and $y$ along the tangent line to the curve at the point in question, rather than along the curve itself. See e.g. https://www.encyclopediaofmath.org/index.php/Differential

The chain rule in two dimensions is counter-intuitive at first if you think of the derivative (and partial derivatives) as ratios:

$$\frac{\mathrm d f}{\mathrm dt} = \frac{\partial f}{\partial x} \frac{\mathrm dx}{\mathrm dt} + \frac{\partial f}{\partial y} \frac{\mathrm dy}{\mathrm dt}.$$

Here, $f$ is a function of two variables, $x$ and $y$, both of which are functions of $t$.

This question has come up at MathOverflow:

Is $\frac{\textrm{d}y}{\textrm{d}x}$ not a ratio?
If $\frac{dy}{dt}dt$ doesn't cancel, then what do you call it?
The rule of thumb to use is to only think in this manner when there is only one independent variable (e.g. when all of your variables are to be thought of as functions of $x$). When all of the variables you are using depend on each other, there a variety of (fully rigorous) ways to think of expressions like “$dx$” as simply being a value with funny units, which cancel out in “$\frac{dy}{dx}$”, resulting in a perfectly ordinary value.
But when there are more variables (e.g. when considering $z = f(x,y)$), this way of thinking no longer works. (Or more accurately, it does still work, but $dx, dy, dz$ behave like vectors so division doesn’t make sense)