For example, this is generally discouraged:
Given that $a$, $b$, and $c$ are sides of a right triangle (where $c$ is the hypotenuse, prove that $a^2+b^2=c^2$.
If you feel that it is somehow just so much more straightforward to copy and paste, then it is generally good real world advice to both quote the question (because you are copying after all) and give a reference to the source (so that others can refer to the context of the question). (For example, if the question is taken from a textbook, please include the title, author, and edition.)