$${\mathtt{U}} = {\mathtt{2}}{\mathtt{\,\times\,}}{\mathtt{r}}{\mathtt{\,\times\,}}{\mathtt{\pi}}$$ $${\mathtt{U}} = {\mathtt{d}}{\mathtt{\,\times\,}}{\mathtt{\pi}}$$
http://web2.0rechner.de/formelsammlung/mathematik/geometrie/kreis
$${\mathtt{U}} = {\mathtt{2}}{\mathtt{\,\times\,}}{\mathtt{r}}{\mathtt{\,\times\,}}{\mathtt{\pi}}$$ $${\mathtt{U}} = {\mathtt{d}}{\mathtt{\,\times\,}}{\mathtt{\pi}}$$
http://web2.0rechner.de/formelsammlung/mathematik/geometrie/kreis