An error with the equation for mutual indutance

[jiajiasun]

On this webpage (https://em.geosci.xyz/content/maxwell3_fdem/circuitmodel_for_eminduction/understanding_harmonicEMresponse.html), the mutual inductance M_12 is derived using Biot-Savart law. But the equation for A_1 is not quite right, because A_1 should be inversely proportional to the distance, instead of the square of the distance. And because of this error, the equations 4 and 5 both have the same problem.

This can be easily verified by looking at the same derivations on Page 311 in David Griffiths' book (Third edition).