Given radius #r# and angle #theta#, the cartesian coordinates #x# and #y# are given by the formulae:
#x = r cos theta#
#y = r sin theta#
Conversely, given #x# and #y#, the radius #r# and angle #theta# are determined by the formulae:
#r = sqrt(x^2+y^2)#
#theta = "atan2"(y, x)#
where #"atan2"(y, x)# is defined as follows:
#"atan2"(y, x) = { (arctan(y/x), " if x > 0"), (arctan(y/x) + pi, " if x < 0 and y >= 0"), (arctan(y/x) - pi, " if x < 0 and y < 0"), (pi/2, " if x = 0 and y > 0"), (-pi/2, " if x = 0 and y < 0"), ("undefined", " if x = 0 and y = 0") :}#