An Operational Amplifier Question (help)

In the figure, why Ia=Ib (use observatinoal method)
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Thanks a lot

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the voltages at the inverting and non-inverting inputs are equal (v-=v+), the two resistors to the supply are also equal (R1=R1) and I assume vI1 and vI2 are equal? hence Ia=Ib.