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Let f be a function dened on an interval 1'. Given two points P and Q on the graph of f 1 we will call mpg the slope of the line going through P and Q. We say that the function f is \"cave up\" on I when for every 3 different points P1 Q, and R 0n the graph of f1 if P is to the left of Q1 and Q is to the left of R, then mP'Q