If $f$ is a real convex function, $x_1, . . . , x_n$ are values in the domain of $f$, and $a_1, . . . , a_n$ are positive real numbers summing to $1$ then
$f(\sum a_ix_i) \leq \sum a_i f(x_i) $.
The function plotted here, showing relationship of radius in cm (horizontal axis) to surface area in $cm^2$ (vertical axis) is a convex function: for any two points on the plotted curve, all internal points on the straight line joining them lie above the curve.
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