>>551
水の体積(π/75)h^3=Vとすると
l=-dV/dt
=-d/dt((π/75)h^3)
=-d/dh((π/75)h^3)・dh/dt
=-(π/25)h^2・dh/dt
∴dh/dt=l/(-(π/25)h^2)=-25l/(πh^2)