Ask Uncle Colin: Some Rigour Required

Integration by substitution, rigorously

Dear Uncle Colin,

Can you explain why integration by substitution works? I get that you’re not allowed to ‘cancel’ the $dx$s, but can’t see how it works otherwise.

– Reasonable Interpretation Got Our Understanding Ridiculed

Hi, RIGOUR, and thanks for your message.

First up, confession time: me and rigour go together like peanut butter and lawnmowers. We are not a natural combination, and any attempt to apply one to the other is likely to end in a mess – or at least anguished screams from @realityminus3. But, since you asked, I’ll do my best.

It helps if we go back to what integration means: $\int y \dx$ gives the family of functions, $F(x)$, such that $\diff {F(x)}{x} = y$.

Also, $x$ is a dummy variable – it wouldn’t matter if you changed it for $t$ or $\theta$ or anything else – it’s just there as a placeholder. In particular, $\int y \d t$ gives a family of functions $F(t)$ which are (up to replacing the variable) the same ones as you’d get from integrating with respect to $x$.

Suppose we have $F(x)$, where $x$ is a function of $t$. Then, if we differentiate $F(x)$ with respect to $t$, we get $\diff {F(x)}{x} \cdot \diff xt$. Integrating this with respect to $t$ – a dummy variable, remember – gets us back to $F(x)$.

So, we have $F(x) = \int y \diff xt \d t$ – but we originally had $F(x) = \int y \dx$, so $\int y \diff xt \d t = \int y \dx$ as required. $\blacksquare$.

Hope that helps!

– Uncle Colin

* Edited 2018-10-03 to fix a typo. Thanks, Adam!

Colin

Colin is a Weymouth maths tutor, author of several Maths For Dummies books and A-level maths guides. He started Flying Colours Maths in 2008. He lives with an espresso pot and nothing to prove.

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2 comments on “Ask Uncle Colin: Some Rigour Required

  • good rat zeke

    * begs for a variable cup of t * >squeaksqueak<

    • Colin

      Good rat, Zeke!

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