If (1-x3 )10= r=010arxr(1-x)30-2r, then 9a9a10 is equal to .
Answer
30
Solution
1 - x3 = (1 - x) (1 + x + x2 ) and 1 + x + x2 = (1 - x)2 + 3x, so
(1 - x3 )10 = (1 - x)10 [(1 - x)2 + 3x ]10
= (1 - x)10 r=01010Cr(3x)r [(1 - x)2 ]10 - r
= r=01010Cr 3r xr(1 - x)30 - 2r
Comparing, ar = 10Cr 3r, so a9 = 10 39 and a10 = 310.
∴ 9a9a10 = 9 10 39310 = 30
Answer: 30