The coefficient of x70 in x2(1 + x)98 + x3(1 + x)97 + x4(1 + x)96 + + x54(1 + x)46 is 99Cp - 46Cq. Then a possible value of p + q is :
- 61
- 83
- 55
- 68
Answer
(2) 83
Solution
The general term is xr(1 + x)100 - r, r = 2, 3, , 54; its coefficient of x70 is 100 - rC70 - r = 100 - rC30
So the required coefficient is
46C30 + 47C30 + + 98C30 = 99C31 - 46C31
(since n = 30NnC30 = N + 1C31, the hockey-stick identity)
As 99C31 = 99C68 and 46C31 = 46C15,
p \31, 68\ and q \31, 15\
Possible values of p + q: 62, 46, 99, 83, of which only 83 is in the options
Answer: (2)