The coefficient of a8b4c9d9 in (abc+abd+acd+bcd)10 is
10!
10!8!4!9!9!
2520
none of these
a10b10c10d101a+1b+1c+1d10Therefore, the required coefficient is equal to the coefficient of a−2b−6c−1d−1 in 1a+1b+1c+1d10, which is given by 10!2!6!1!1!=10×9×8×72=2520