Bài 13.
Mạch 1: $r // r // r$
$\dfrac{1}{R_{tđ}} = \dfrac{1}{r} + \dfrac{1}{r} + \dfrac{1}{r}$
$\to R_{tđ} = \dfrac{r}{3}$
Mạch 2: $r // (r nt r) // r$
$\dfrac{1}{R_{tđ}} = \dfrac{1}{r} + \dfrac{1}{2r} + \dfrac{1}{r} = \dfrac{5}{2r}$
$\to R_{tđ} = \dfrac{2r}{5}$