Prove that$ f\Bigg(\sum_{i=1}^n \lambda_i x_i\Bigg) \leq \sum_{i=1}^n \lambda_i f(x_i) $ for a convex function "f"
Show that $f$ is convex if and only if $f\left( \sum_{i=1}^m\lambda_ix_i \right) \leq \sum_{i=1}^m\lambda_if(x_i)$