The electrical resistance $\>R\>$ of a certain wire is given by $\>R = \frac {k}{r^2}\>$, where $\>k\>$ is a constant and $\>r\>$ is the radius of the wire. Assuming that the radius $\>r\>$ has a possible error of $\> \pm 9 \%$, use differentials to estimate the percentage error in $\>R$.

Error: $\pm$ %

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