x^2 + (y-2)^2 = 4 is a circle centered at (0,2) with radius=2, touching the x-axis
when converting this circle to a polar equation in term of r and θ:
r is a chord w/ one end at (0,0)
θ is a tangent chord angle formed by the x-axis and r
by properties of tangent chord angle, r=2*radius*sinθ
r=4sinθ
