Hi,

The formula used in section 2.1 is derived by nodal analysis in which we see the bias current causing a voltage drop across Rf. However, the same result can be obtained by the method shown in section 2.2. That is, multiplying Ib by the parallel equivalent of the feedback network gives us the input referred voltage offset due to the bias current. Similar to Vos, this input referred offset is multiplied by the non-inverting gain of the circuit, resulting in the output voltage offset due to the bias current. Try setting the two equations equal to each other and you will see that all of the terms cancel out.

Zach Olson