and it is easy to show that x = 1 is a global minimum to f, so
f(x) ≥ f(1) = 0.
Another proof is based on the Taylor expansion of the exponential function, yielding
t2
et =1+t+ 2 ·eθ,whereθ∈(0,t).Putt=x−1,and(3)follows.
The continuous arithmetic, geometric and harmonic means of positive, integrable function f : [a, b] → R are defined by