>>148 the d is a "differential" which is a very tiny change. If you start with
Δf(x)/Δx then the limit of that ratio as Δx goes to zero is d(f(x))/dx (which I
have usually seen written as either df/dx or dy/dx). But the whole d/d notation is an operator that stands for lim Δx->0 Δf(x)/Δx.