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

theorem Th10:
  cosh_C/.z = cosh_C/.(-z)
proof
  cosh_C/.(-z) = (exp(-z) + exp(-(-z)))/2 by Def4
    .= (exp(-z) + exp(z))/2;
  hence thesis by Def4;
end;
