What is the expansion of -LRB- 1 + x -RRB- ^ -1 ?
How can we find the coefficient of -LSB- math -RSB- x ^ 3 -LSB- / math -RSB- in the expansion of -LSB- math -RSB- -LRB- 1 + x + x ^ 2 -RRB- ^ n -LSB- / math -RSB- ?