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September 15, 2014

September 15, 2014

Posted by **Kim** on Wednesday, March 14, 2012 at 11:25pm.

I know A=4pir^2 and A' is 8pir but I'm not sure how to set it up from here. Can someone please show me all the steps involved? (This isn't HW, just a textbook question that I can't figure out, thanks!)

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**Kim**, Thursday, March 15, 2012 at 1:09amHa! I figured it out.

dA=8pir dr

r=10 , dr=10.2

dA=8(10)(.2)pi

dA=16pi (approx)

check:

A(10)=4(10)^2pi

A(10)=400pi

A(10.2)=4(10.2)^2pi

A(10.2)=416.16

A(10.2)-A(10)=16.16pi (exact)

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