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## Homework Statement

A spherical raindrops evaporates at a rate proportional to its surface area. If its radius is 3mm, and 1 hour later has been reduced to 2mm, find an expresssion for the raduis of the raindrops at anytime.

## Homework Equations

[tex]Volume = \frac{4}{3}\pi R^3[/tex]

[tex]Area = 4\pi R^2[/tex]

## The Attempt at a Solution

[tex]\frac{d}{dt}(\frac{4}{3}\pi R^3) = -k (4\pi R^2)[/tex]

[tex]4\pi R^3 \frac{d}{dt} = -k 4\pi R^2[/tex]

[tex]\frac{R dR}{dt} = -k[/tex]

then... how do I plug in 2 and 3 mm??