Proof is on Introduction to Algorithm
proof is easy to understand


if does not meet regularity condition, you can’t use the master theorem in the third case
Example

Master theorem (analysis of algorithms)
In the analysis of algorithms, the master theorem for divide-and-conquer recurrences provides an asymptotic analysis (using Big O notation) for recurrence relations of types that occur in the analysis of many divide and conquer algorithms. The approach was first presented by Jon Bentley, Dorothea Blostein (née Haken), and James B. Saxe in 1980, where it was described as a "unifying method" for solving such recurrences.[1] The name "master theorem" was popularized by the widely-used algorithms textbook Introduction to Algorithms by Cormen, Leiserson, Rivest, and Stein.
https://en.wikipedia.org/wiki/Master_theorem_(analysis_of_algorithms)

Seonglae Cho