Hey Kohoutek.

Can you show us what you have tried? The basic definition of convexity is that f(at + (1-t)b) is in the codomain of the function (i.e. the values that actually get output from the function), where a and b are just points in the domain of the function and t is a scalar number ranging from 0 to 1 inclusive.

If a function is defined everywhere and takes on every possible value then it will be convex.