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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=