Question

What is the derivative of `2*pi*r`?

Answer 1

`(d(2pir))/(dr)`
`color(white)("XXX")=2pi(dr)/(dr)` by the Constant Rule for Derivatives

`color(white)("XXX")=2pi`

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The Constant Rule for Derivatives tells us that
If `f(x)=c * g(x)` for some constant `c`
then `f'(x)=c * g'(x)`

In this case `f(r)=2pir; c=2pi, and g(r)=r`