i) If we pick any 2 points a and b on such a curve, for a < x < b, f(x) < corresponding value on the line segment ab. In particular the value of the mid-point of ab > f(mid-point of ab) = f([a+b]/2). i.e.
ii) In the same way:
But
Edit:
Turns out to be more or less equivalent to seanieg89's suggested approach.