How can the integral be rewritten using a change of variables?

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The integral transformation discussed is a standard application of the change of variables technique in calculus. Specifically, the equation presented is ∫ f(a(t)) dt = ∫ f(a) (dt/da) da, which illustrates how to rewrite an integral by substituting a new variable. This method is essential for simplifying integrals and is widely used in various fields, including physics and engineering. The reference provided leads to a lecture on cosmology that further elaborates on this concept.

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Here http://www.damtp.cam.ac.uk/user/db275/Cosmology/Lectures.pdf, I find on page 31 in (2.1.5)
distance.PNG


I assume that it is childish calculus that connects both sides of this equation. But still, can someone help me why the integral can be rewritten like that?

thanks!
 
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It is a standard change of variables.

##\int f(a(t)) dt = \int f(a) (dt/da) da##
 
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