Ben R. answered 09/02/19
PhD in Mathematics, 13 years college teaching experience
I think the easiest way to start is to break the larger function into its pieces:
f(1+Δx) = 1/(1+Δx)
f(1) = 1/1 = 1
Then combine them, getting common denominators:
1/(1+Δx) - 1 = 1/(1+Δx) + (1+Δx)/(1+Δx) = (-Δx)/(1+Δx)
If we take this and divide by Δx, we get (-1)/(1+Δx)