Answer: n-r.
- A n+r
- B r-1
- C n-r
- D r
Correct answer: C. n-r
Explanation: Symmetry property: nCr = nC(n-r). For example, 5C2 = 5C3 = 10.
Row n of Pascal's triangle gives the coefficients nC0, nC1, ..., nCn for the expansion of (a+b)ⁿ; each number is the sum of the two numbers diagonally above it, a direct visual proof of the identity nCr = (n-1)C(r-1) + (n-1)Cr.
Concept context
Expansion of (a+b) n , general term, middle term, binomial coefficients, and greatest term