Given a function f(x) on R, and that f(x) is strictly increasing and strictly concave: f'(x) > 0, and f''(x) <0. Is it always true that, for such function, we have: f(a+b) < f(a) + f(b) a,b are real numbers.

Find the local maximum and minimum values and saddle points of the function. If you have three-dimensional graphing software, graph the function with a domain and viewpoint that reveal all the important aspects of the function
$f(x,y)=x^3-6xy+8y^3$

$\frac{1}{\sec <!--\; \; -->(x)}$ in derivative?

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