Although copyright is a powerful tool for creators, there are exceptions to its provisions. Those can range from exceptions applied by the creator (Creative Commons), to exceptions due to age (Public Domain) to exceptions due to how the work will be used (Fair Use). It is important to understand these exceptions so that you use works in an ethical and legal manner.
If you find yourself using a mathematical formula to determine if you can use someone else's work, STOP! The law does not recognize a mathematical application of exceptions.