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

theorem Th21:
  sin_C/.0c = 0
proof
  sin_C/.0c = (exp(0c)-exp(-<i>*0c))/(<i>*2) by Def1
    .=0c/(<i>*2);
  hence thesis;
end;
