If the constant term in the expansion of (1 + 2x - 3x3 ) ( 32x2 - 13x )9 is p, then 108p is equal to .
Answer
54
Solution
Tr+1 = 9Cr ( 32 )9-r (- 13 )r x18 - 3r
18 - 3r is a multiple of 3, only x0 (r = 6) and x-3 (r = 7) pair with 1 and -3x3; 2x gives none
r = 6: 9C6 278 1729 = 84216 = 718
r = 7: 9C7 94 (- 12187 ) = - 127
p = 718 + (-3) (- 127 ) = 718 + 19 = 12 108p = 54
Answer: 54