Let f ( x ) = x 9 7 + 9 x 2 7 2 . {\displaystyle \displaystyle f(x)=x^{\frac {9}{7}}+{\frac {9x^{\frac {2}{7}}}{2}}.} (c) Find the intervals on which f ( x ) {\displaystyle f(x)} is concave up and the intervals on which f ( x ) {\displaystyle f(x)} is concave down.
