reserve a,b,c,x,y,z for object,X,Y,Z for set,
  n for Nat,
  i,j for Integer,
  r,r1,r2,r3,s for Real,
  c1,c2 for Complex,
  p for Point of TOP-REAL n;

theorem Th10:
  cos(r) = cos(s) implies ex i st r = s + 2*PI*i or r = -s + 2*PI*i
  proof
    assume
A1: cos(r) = cos(s);
A2: cos(r) - cos(s) = -2*(sin((r+s)/2)*sin((r-s)/2)) by SIN_COS4:18;
    per cases by A1,A2;
    suppose sin((r-s)/2) = 0;
      hence thesis by Lm5;
    end;
    suppose sin((r+s)/2) = 0;
      then consider i such that
A3:   (r+s)/2 = PI*i by Th7;
      r+s-s = 2*PI*i-s by A3;
      hence thesis;
    end;
  end;
