Suppose 2-p, p, 2- , are the coefficients of four consecutive terms in the expansion of (1+x)n. Then the value of p2- 2+6 +2p equals
- 8
- 4
- 6
- 10
Answer
BONUS
Solution
Let nCr=2-p, nCr+1=p, nCr+2=2- , nCr+3=
nCr+nCr+1=n+1Cr+1=2
and nCr+2+nCr+3=n+1Cr+3=2
n+1Cr+1=n+1Cr+3 (r+1)+(r+3)=n+1 n=2r+3
2r+4Cr+1=2; but NCk=2 only for N=2, k=1, i.e. r=-1: impossible
∵ no such n, r exist, the data are inconsistent ⇒ question dropped
Answer: Bonus (full marks to all who attempted)