I am very interested in dating rocks and am about to read the book of Gunter Faure, Principles of Isotope Geology. I came across the linear equation below, which I understand well.

(87Sr/86Sr)t = (87Sr/86Sr)i + (87Rb/86Sr)t * lambda * t

where the index t after the parenthesis refers to time dependency and i to the initial value, lambda is the decay constant.

However, I don't understand the interpretation that is given in the book, that is, the claim that (87Rb/86Sr)t * lambda is the slope. How can this be since a slope in linear equation must be independent of the variable t, which is not the case. In fact rubidium 87 decays to stable strontium 87 so (87Rb/86Sr)t is decreasing over time since strontium 86 is stable as well. Can someone explain this oddity?