Current location - Training Enrollment Network - Mathematics courses - I want to ask a question about gre mathematical permutation and combination (with Chinese translation questions), and I hope I can explain the process in detail.
I want to ask a question about gre mathematical permutation and combination (with Chinese translation questions), and I hope I can explain the process in detail.
If two of the three letters are the same, it is equivalent to choosing two letters from 26 alphabets (in no particular order), that is,
And the order of the three letters is abb, bab and bba, so