Replace repeated derivation with classification.

The Master Theorem is a shortcut for determining the time complexity of many divide-and-conquer algorithms. It applies to recurrences where a problem is split into equally sized subproblems, each solved recursively, with additional work performed to divide and combine the results. By comparing the cost of the recursive calls with the cost of work done at each level, the theorem quickly identifies the dominant growth rate. It turns what could be a tedious recurrence calculation into a compact classification problem. Algorithms such as merge sort fit naturally into its framework. It is used primarily in algorithm analysis, divide-and-conquer design, and computer science education.