//unärer additionsoperator += addition;  bruch+=bruch2 -> bruch=bruch+bruch2;
rational rational::operator+=(rational const& bruch2)
{
	rational tmp=rational((zaehler_*bruch2.nenner_)+(nenner_*bruch2.zaehler_),(nenner_*bruch2.nenner_));
	*this = tmp;
	return *this;
}

//normale Addition zweier brüche
rational operator+(rational const& bruch1,rational const& bruch2)
{	rational tmp=bruch1;
	return (tmp+=bruch2);
}