Electrical engineering - equivalent resistance
\$R_5\$ is between \$A\$ and \$B\$

Since they are all the same resistance, no current will flow through \$R_4\$. This means that I can remove \$R_4\$ from the circuit. 

\$R_0 + R_1\$ is between \$A\$ and \$B\$

\$R_2 + R_3\$ is between \$A\$ and \$B\$

So you have 3 branches in parallel. 

The total resistance will be:

$$\frac{1}{\frac{1}{R_0+R_1}+\frac{1}{R_2+R_3}+\frac{1}{R_5}} = 0.5 Ω$$



Why can I say that no current will flow through \$R_4\$?

That is because the potential across \$R_4\$ will be 0 Volt. The node between \$R_0\$ and \$R_1\$ will have the exact same potential as the node between \$R_2\$ and \$R_3\$.

If we make use of ohm's law \$V=I×R\$, we have \$V=0\$ and \$R=R_4=1Ω\$. So let's move the equation around so we can find \$I\$. 

\$I = \frac{V}{R}=\frac{0 V}{1Ω} = 0\$ A

There is no current flowing through that resistor, that means that if the resistance would be \$\infty\$ Ω, it would also be \$0\$ A flowing through that resistor. 

Do you know what else is \$\infty\$ Ω? Something that is not connected = not part of the circuit.