Question 1191911
A circular ripple spreads across a lake. If the area of the ripple increases 
at a rate of 10pi m^2s^-1,<pre>
That means {{{dA/dt=10pi}}}{{{m^2/s}}}

We want a formula for the area of a circle.

{{{A=pi*r^2}}}
{{{dA/dt=2*pi*r*expr(dr/dt)}}}
Substitute {{{10pi}}} for {{{dA/dt}}} and simplify</pre>
find the rate at which the radius is increasing when the radius is 2 m.<pre>
Substitute 2 for r
Solve for {{{dr/dt}}}

Edwin</pre>