So my question is-

A fire has started in a dry open field and is spreading in the form of a circle. If the radius of this circle increases at the rate of 6 feet/ minute express the total area A as a function of time t (in minutes).

It seems to me that area would = time(6(pi)time^2)

The answer in the book says the answer is= 36(pi)time^2

It is not clear to me why the 6 must be squared.

Help?

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