Help with log proof pls
In picture below is proof of area
under y = 1/x. I understand everything, but second part of step to, which is 1/z dz = L(b). In explanation they are just substituting z with x, but how is it possible, If earlier it's said that x = az, not x = z.
![Help with log proof pls In picture below is proof of area under y 1x I understand everything but second part of step to which is 1z dz Lb In explanation they ar class=](https://us-static.z-dn.net/files/d3a/545381442b5bda09e6cf40df957728ea.jpg)