Expand (A + B)ⁿ term by term and find any single term.
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Why it works
Each term of (A+B)ⁿ picks A or B from each of n factors; choosing B exactly k times can be done C(n,k) ways — that counting argument is why Pascal’s triangle coefficients appear. The (k+1)th term is C(n,k)Aⁿ⁻ᵏBᵏ.
