Problem: The coefficient of x7 in the polynomial expansion of (1+2x−x2)4 is
Answer Choices:
A. −8
B. 12
C. 6
D. −12
E. none of these
Solution:
The coefficient of x7 in (1+2x−x2)4 is the coefficient of the sum of four identical terms 2x(−x2)3, which sum −8x7.