exercise for the reader - when found in a textbook, implies that the task in question is trivial. Ex. "Step 1: Gather writing paper. Step 2: sharpen pencil. Step 3: Write the Great American Novel. I shall leave the details of this last step as an exercise for the reader."

Or, better yet:

Now, we know that given A, B, and C, and some positive integer n > 2, A^n + B^n = C^n, and from this, we find that i = 5.4.

The proof of this is left as an exercise for the reader. It should take one hour to complete this task. Please remember value of n - it will be on your final.

Related phrase:

It can be proved that (idea)

Phrase applied as a prefix to a statement, expressing "I am not going to supply evidence, because this is a well established, obvious fact and the proof is so exceedingly trivial that it's too boring for intelligent people like us to expound further on it and anyone who doesn't believe me is an idiot." Used to cover that one is absolutely unable to prove said statement.