find $\displaystyle R_{ab}$

first I added $\displaystyle 5\Omega + 5\Omega = 10\Omega$

then $\displaystyle \frac{10\Omega\times10\Omega}{10\Omega+10\Omega}=5 \Omega$

then $\displaystyle \frac{8\Omega\times8\Omega}{8\Omega+8\Omega}=4\Ome ga$

so $\displaystyle 4\Omega+ 5\Omega+ 1\Omega = \boxed{1\Omega}$

any not really sure about this