reserve x,y for Real;
reserve z,z1,z2 for Complex;
reserve n for Element of NAT;

theorem Th23:
  cos_C/.0c = 1
proof
  cos_C/.0c = (exp(0c)+exp(-<i>*0c))/2 by Def2
    .= 1 by SIN_COS:49,51;
  hence thesis;
end;
