Answer: 5.
- A 3
- B 6
- C 4
- D 5
Correct answer: D. 5
Explanation: nC<sub>1</sub> : nC<sub>2</sub> = 1:2. n/(n(n-1)/2) = 1/2. 2/(n-1) = 1/2. n-1 = 4, n = 5.
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