Question Number 141628 by mathdanisur last updated on 21/May/21
Commented by MJS_new last updated on 21/May/21
$$\mathrm{what}\:\mathrm{are}\:{r},\:{r}_{{a}} ,\:...? \\ $$$${rr}_{{a}} \:\mathrm{means}\:{r}×{r}_{{a}} ? \\ $$
Commented by mathdanisur last updated on 21/May/21
$${yes}\:{dear}\:{Sir} \\ $$
Commented by mitica last updated on 21/May/21
$${r}_{{a}} ={ptg}\frac{{A}}{\mathrm{2}} \\ $$