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
(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
a9=10 39, a10=310
9a9a10= 9 10 39310=30
Answer: 30