theorem Th8:
  cos(r) = 0 implies ex i st r = PI/2+PI*i
  proof
    assume cos(r) = 0;
    then r = PI/2+2*PI*[\r/(2*PI)/] or r = 3*PI/2+2*PI*[\r/(2*PI)/] by Th6;
    then r = PI/2+PI*(2*[\r/(2*PI)/]) or r = PI/2+PI*(1+2*[\r/(2*PI)/]);
    hence thesis;
  end;
