I am working on this question:
Let f and g be two strictly concave functions and let function $h$ be defined by $h = af + bg$, where $ a > 0$ and $ b > 0$ are constants. Using the definition of strict concavity (i.e., without the use of derivatives) show that $h$ is strictly concave.
How can I show that $h$ is strictly concave? Should I show that because both $af$ and $bg$ are increasing then their sum must also be increasing?