Answer: False.
Explanation:
A corollary is the statement that follows with no or small proof required from an already proven statement or theorems.
Its proof is not difficult because it is based on the statement or theorem which has been already proved.
For example the fundamental theorem of algebra and its corollary.
Therefore, the statement "A corollary is a statement that can be proved using a theorem, but the proof is usually difficult." is false.