The coefficient of x20 in the expansion of 1+11!x+12!x2+…+120!x203is
0
320!
32020!
60C20
Note that, coefficient of x20 in 1+11!x+12!x2+…+120!x203
coefficient of x20 in 1+11!x+12!x2+…+120!x20+121!x21+…3
coefficient of x20 in e3x=32020!