Calculating Percent Uncertainty of Circle Area w/ Radius 1.8x10^4 cm

[future_vet]

Homework Statement

What is the percent uncertainty in the area of a circle whose raduis is 1.8 x 10^4 cm?

Homework Equations

I calculated A=Pi x r^2 = 1017876020
After that, I am lost...

The Attempt at a Solution

Thanks!

 

[future_vet]

The thing is, I don't know what was used to measure the radius. Can I still know what the uncertainty is to calculate the %age?

Thanks!

 

[jtbell]

If I'm not given any direct information about the uncertainty in the initial measurement (here, the radius), I take it to be +/- 1 in the last significant figure. In this case the uncertainty in the radius would be $0.1 \times 10^4$ cm.

 

[future_vet]

Thanks a lot!

I calculated the percent uncertainty to be 5.6%. I think it is correct...

 

[future_vet]

Apparently this is not the correct answer! please help me ...

Thanks!

 

[future_vet]

Ok it was 11%... I don't understand why though... If you could walk me through the thinking process, I'd be grateful. I will know how to do such a problem next time... Thanks!