x0 =  integral b
ar(f) cos f V~ --------------
  r(f)2 + r'(f)2 df /s, y0 =  integral  b
 ar(f) sin f V~ --------------
  r(f)2 + r'(f)2 df /s, °missä s =  integral b
a V~ r(f)2+-r'(f)2 df.