# Inverse function problem

#### dbag

Hello
I have a folllowing question
Function A(r)=2*pi*r2+6*pi*r. I has a an inverse function r(A)=(-6*pi+-sqrt(4*pi(9*pi+2r))/4*pi
Calculate at what radius does the area of the cylinder equal 200. do i just make the function r(A)=200? Thanks

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#### Idea

it should be

$$\displaystyle r=\frac{-6 \pi + \sqrt{4\pi (9 \pi +2A)}}{4 \pi }$$

now replace $A=200$

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2 people

#### TKHunny

I has a an inverse function r(A)=(-6*pi+-sqrt(4*pi(9*pi+2r))/4*pi
This is a very bad mistake. Understanding what a function is and isn't is SO fundamental. Please review the definition.

1 person