calaulos~~~也很急!!!!!!

A=πr^2
dA/dt
=(2πr)(dr/dt)
since the area of a circular plate is increasing at the same rate as its radius increasing
So let dA/dt=dr/dt=x
dA/dt=(2πr)(dr/dt)
x=(2πr)x
1=2πr
r=1/2π
the radius at that instant is 1/2π