A lesser-known (but extremely useful) integration trickpic.twitter.com/20Qgs8QeNk
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Brilliant! How did you make this gif?
Matemathica
I'd visualize the second animation as going right-to-left (from b to a) bc as x goes from a to b, (a+b-x) goes from b to a. But that's just me.
But then, the function itself wouldn't be reversed, so...
Wait. What does the area have to do with the integrand? ...... 
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