`Cylinder = line-segment `

`×`

disc

`C = | × ●`

The “product rule” from calculus works as well with the boundary operator `∂`

as with the differentiation operator `∂`

.

`∂C = ∂| × ● + | × ∂●`

>Oops. Typo. Sorry, I did this really late at night! cos and sin need to be swapped back.

Oops. Another typo. Wrong formula for circumference.

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