In the expansion of (9x- 13 x )18, x>0, if the term independent of x is (221)k, then k is equal to :
- 84
- 78
- 168
- 198
Answer
(1) 84
Solution
Tr+1=18Cr(9x)18-r (- 13 x )r
=18Cr (-1)r 336-2r 3-r x18-r- r2=18Cr (-1)r 336-3r x18- 3r2
Independent of x: 18- 3r2=0 r=12
T13=18C12 30=18C6= 18 17 16 15 14 13720=18564
18564=221 84
k=84
Answer: (1)