Let Cr denote the coefficient of xr in the binomial expansion of (1 + x)n, n N, 0 r n. If Pn = C0 - C1 + 223C2 - 234C3 + + (-2)nn+1Cn, then the value of n=125 1P2n equals
- 675
- 580
- 525
- 650
Answer
(1) 675
Solution
General term: Pn = r=0n (-2)rr+1Cr
Crr+1 = 1n+1 n+1Cr+1
Pn = 1n+1 r=0n n+1Cr+1(-2)r = 1-2(n+1) k=1n+1 n+1Ck(-2)k
= (1 - 2)n+1 - 1-2(n+1) = 1 - (-1)n+12(n+1)
For 2n in place of n: P2n = 1 + 12(2n+1) = 12n+1
n=125 1P2n = n=125(2n + 1) = 2 25 262 + 25
= 650 + 25 = 675
Answer: (1)