After the switch is closed, the circuit can be simplified as a circuit in which the upper two resistors are connected in parallel, the lower two resistors are connected in parallel, and then the two parts are connected in series.

The total resistance of the two parallel parts is the same, and the voltage of each part is 9V.

Above: The 3 Ω resistance current is 3A, and the 6 Ω resistance current is1.5a.

Below: The 3 Ω resistance current is 3A, and the 6 Ω resistance current is1.5a.

It can be seen that only 1.5A of the upper 3 Ω resistor current flows to the lower 6 Ω resistor, and 1.5A flows to the lower 3 Ω resistor through AB.

All current from the upper 6 Ω resistor flows to the lower 3 Ω resistor.

Therefore, iab =1.5a.