x = r·cos(theta)
y = r·sin(theta)
x2 + (y + 5)2 = 25
( r·cos(theta) )2 + ( r·sin(theta) + 5 )2 = 25 (substitue)
r2·cos2(theta) + r2·sin2(theta) + 10·r·sin(theta) + 25 = 25 (multiply out)
r2·cos2(theta) + r2·sin2(theta) + 10·r·sin(theta) = 0 (subtract 25)
r2·[ cos2(theta) + sin2(theta) ] + 10·r·sin(theta) = 0 (factor)
r2·[ 1 ] + 10·r·sin(theta) = 0 (replace sin2 + cos2 with 1)
r2·+ 10·r·sin(theta) = 0
r·[ r + 10·sin(theta) ] = 0 (factor)
Either r = 0 or r + 10·sin(theta) = 0
---> r = -10·sin(theta)